WordNet

duplicate or match; "The polished surface twinned his face and chest in reverse" (同)duplicate, parallel
give birth to twins
grow as twins; "twin crystals"
either of two offspring born at the same time from the same pregnancy
derived from two separately fertilized eggs; "dizygotic twins" (同)dizygous
(mineralogy) two interwoven crystals that are mirror images on each other

PrepTutorEJDIC

『双生児の一人』;《複数形で》双生児,ふたご / よく似た人(物)の片方;《複数形で》対 / ふたごの;対の / 〈ある都市〉‘を'(…と)姉妹関係にする《+名+with+名》

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/06/21 00:14:05」(JST)

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Marian and Vivian Brown, identical twins, photographed by Christopher Michel

Twins are two offspring produced by the same pregnancy.^[1] Twins can either be monozygotic ("identical"), meaning that they can develop from just one zygote that will then split and form two embryos, or dizygotic ("fraternal"), meaning that they can develop from two different eggs, each are fertilized by separate sperm cells.^[2]

In contrast, a fetus which develops alone in the womb is called a singleton, and the general term for one offspring of a multiple birth is multiple.^[3]

1 Statistics
2 Zygosity
- 2.1 Dizygotic (fraternal) twins
- 2.2 Monozygotic (identical) twins
  - 2.2.1 Mechanism
  - 2.2.2 Incidence
  - 2.2.3 Genetic and epigenetic similarity
3 Degree of separation
4 Demographics
- 4.1 Ethnicity
- 4.2 Predisposing factors
5 Delivery interval
6 Complications during pregnancy
- 6.1 Vanishing twins
- 6.2 Conjoined twins
- 6.3 Chimerism
- 6.4 Parasitic twins
- 6.5 Partial molar twins
- 6.6 Miscarried twin
- 6.7 Low birth weight
- 6.8 Twin-to-twin transfusion syndrome
7 Management of birth
8 Human twin studies
- 8.1 Unusual twinnings
- 8.2 Semi-identical twins
- 8.3 Mirror image twins
9 Language development
10 Other animals
11 See also
12 References
13 Further reading
14 Foundations

Statistics

The twin birth rate in the United States rose 76% from 1980 through 2009, from 18.9 to 33.3 per 1,000 births.^[4] The Yoruba have the highest rate of twinning in the world, at 45–50 twin sets (or 90–100 twins) per 1,000 live births,^[5]^[6]^[7] possibly because of high consumption of a specific type of yam containing a natural phytoestrogen which may stimulate the ovaries to release an egg from each side.^[8]^[9]

In Central Africa there are 18–30 twin sets (or 36–60 twins) per 1,000 live births.^[10] In Latin America, South Asia, and Southeast Asia, the lowest rates are found; only 6–9 twin sets per 1,000 live births. North America and Europe have intermediate rates of 9–16 twin sets per 1,000 live births.^[10]

Multiple pregnancies are much less likely to carry to full term than single births, with twin pregnancies lasting only 37 weeks (3 weeks less than full term) on average.^[11]

Women who have a family history of fraternal twins have a higher chance of producing fraternal twins themselves, as there is a genetically linked tendency to hyper-ovulate. There is no known genetic link for identical twinning.^[12] Other factors that increase the odds of having fraternal twins include maternal age, fertility drugs and other fertility treatments, nutrition, and prior births.^[13]

Zygosity

Zygosity is the degree of identity in the genome of twins. There are five common variations of twinning.

The three most common variations are all dizygotic (fraternal):^[14]

Male–female twins are the most common result, 50 percent of dizygotic twins and the most common grouping of twins.
Female–female dizygotic twins (sometimes called "sororal twins")
Male–male dizygotic twins

The other two variations are monozygotic ("identical") twins:^[14]

Female–female monozygotic twins
Male–male monozygotic twins (less common)

Among non-twin births, male singletons are slightly (about five percent) more common than female singletons. The rates for singletons vary slightly by country. For example, the sex ratio of birth in the US is 1.05 males/female,^[15] while it is 1.07 males/female in Italy.^[16] However, males are also more susceptible than females to die in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins.

Dizygotic (fraternal) twins

Eight-month-old sororal twins napping

Dizygotic (DZ) or fraternal twins (also referred to as "non-identical twins", "dissimilar twins", "biovular twins", and, informally in the case of females, "sororal twins") usually occur when two fertilized eggs are implanted in the uterus wall at the same time. When two eggs are independently fertilized by two different sperm cells, fraternal twins result. The two eggs, or ova, form two zygotes, hence the terms dizygotic and biovular. Fraternal twins are, essentially, two ordinary siblings who happen to be born at the same time, since they arise from two separate eggs fertilized by two separate sperm, just like ordinary siblings. Instead of fraternal twins, this situation can also occasionally result in a chimera, someone colloquially described as their own fraternal twin.

Dizygotic twins, like any other siblings, have an extremely small chance of having the same chromosome profile. Even if they happen to have the same chromosome profile, they will always have different genetic material on each chromosome, due to chromosomal crossover during meiosis. Like any other siblings, dizygotic twins may look similar, particularly given that they are the same age. However, dizygotic twins may also look very different from each other. They may be of different sexes or the same sex, just like ordinary siblings.

Studies show that there is a genetic basis for dizygotic twinning. However, it is only the mother who has any effect on the chances of having such twins; there is no known mechanism for a father to cause the release of more than one ovum. Dizygotic twinning ranges from six per thousand births in Japan (similar to the rate of monozygotic twins) to 14 and more per thousand in some African countries.^[8]

Dizygotic twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35.^[17] With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly.

Monozygotic (identical) twins

Comparison of zygote development in monozygotic and dizygotic twins. In the uterus, a majority of monozygotic twins (60–70%) share the same placenta but have separate amniotic sacs. In 18–30% of monozygotic twins each fetus has a separate placenta and a separate amniotic sac. A small number (1–2%) of monozygotic twins share the same placenta and amniotic sac. Fraternal twins each have their own placenta and own amniotic sac.

Monozygotic (MZ) or identical twins occur when a single egg is fertilized to form one zygote (hence, "monozygotic") which then divides into two separate embryos.

Mechanism

Regarding spontaneous or natural monozygotic twinning, a recent theory proposes that monozygotic twins are formed after a blastocyst essentially collapses, splitting the progenitor cells (those that contain the body's fundamental genetic material) in half, leaving the same genetic material divided in two on opposite sides of the embryo. Eventually, two separate fetuses develop.^[18] Spontaneous division of the zygote into two embryos is not considered to be a hereditary trait, but rather a spontaneous or in some cases a random event.^[17]^[19]

Monozygotic twins may also be created artificially by embryo splitting. It can be used as an expansion of IVF to increase the number of available embryos for embryo transfer.^[20]

Incidence

Monozygotic twinning occurs in birthing at a rate of about three in every 1000 deliveries worldwide.^[19]

The likelihood of a single fertilization resulting in monozygotic twins is uniformly distributed in all populations around the world.^[17] This is in marked contrast to dizygotic twinning, which ranges from about six per thousand births in Japan (almost similar to the rate of identical twins, which is around 4–5) to 15 and more per thousand in some parts of India^[21] and up to over 20 in some Central African countries.^[10] The exact cause for the splitting of a zygote or embryo is unknown.

In vitro fertilization (IVF) techniques are more likely to create dizygotic twins. For IVF deliveries, there are nearly 21 pairs of twins for every 1,000.^[22]

Genetic and epigenetic similarity

Monozygotic twins are genetically nearly identical and they are always the same sex unless there has been a mutation during development. The children of monozygotic twins test genetically as half-siblings (or full siblings, if a pair of monozygotic twins reproduces with another pair or with the same person), rather than first cousins. Identical twins do not have the same fingerprints, due to the fact that even in a small space inside the womb, people have contact with different parts of this environment, which gives small variations in the same digital, making them unique.^[23] Monozygotic twins always have different phenotypes. Normally due to an environmental factor or the deactivation of different X chromosomes in female monozygotic twins, and in some extremely rare cases, due to aneuploidy, twins may express different sexual phenotypes, normally from an XXY Klinefelter syndrome zygote splitting unevenly.^[24]^[25]^[26]

Monozygotic twins, although genetically very similar, are not genetically exactly the same. The DNA in white blood cells of 66 pairs of monozygotic twins was analyzed for 506,786 single nucleotide polymorphisms known to occur in human populations. Polymorphisms appeared in 2 of the 33 million comparisons, leading the researchers to extrapolate that the blood cells of monozygotic twins may have on the order of one DNA-sequence difference for every 1.2 x 10⁷ nucleotides, which would imply hundreds of differences across the entire genome.^[27] The mutations producing the differences detected in this study would have occurred during embryonic cell-division (after the point of fertilization). If they occur early in fetal development, they will be present in a very large proportion of body cells.

Another cause of difference between monozygotic twins is epigenetic modification, caused by differing environmental influences throughout their lives. Epigenetics refers to the level of activity of any particular gene. A gene may become switched on, switched off, or could become partially switched on or off in an individual. This epigenetic modification is triggered by environmental events. Monozygotic twins can have markedly different epigenetic profiles. A study of 80 pairs of monozygotic twins ranging in age from three to 74 showed that the youngest twins have relatively few epigenetic differences. The number of epigenetic differences increases with age. Fifty-year-old twins had over three times the epigenetic difference of three-year-old twins. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference.^[28] However, certain characteristics become more alike as twins age, such as IQ and personality. This phenomenon illustrates the influence of genetics in many aspects of human characteristics and behavior.^[29]^[30]^[31]

A 1981 study of a deceased triploid XXX twin fetus without a heart showed that although its fetal development suggested that it was an identical twin, as it shared a placenta with its healthy twin, tests revealed that it was probably a polar body twin. The authors were unable to predict whether a healthy fetus could result from a polar body twinning.^[32] In 2003, a study argued that many cases of triploidity arise from semi-identical twinning.^[33] In 2007, a study reported a case of a pair of living twins, one intersex and one a phenotypical male. The twins were both found to be chimeras and to share all of their maternal DNA but only half of their father's DNA. The exact mechanism of fertilization could not be determined but the study stated that it was unlikely to be a case of polar body twinning.^[34]

Degree of separation

Various types of chorionicity and amniosity (how the baby's sac looks) in monozygotic (one egg/identical) twins as a result of when the fertilized egg divides

The degree of separation of the twins in utero depends on if and when they split into two zygotes. Dizygotic twins were always two zygotes. Monozygotic twins split into two zygotes at some time very early in the pregnancy. The timing of this separation determines the chorionicity (the number of placentae) and amniocity (the number of sacs) of the pregnancy. Dichorionic twins either never divided (i.e.: were dizygotic) or they divided within the first 4 days. Monoamnionic twins divide after the first week.

In very rare cases, twins become conjoined twins. Furthermore, there can be various degrees of shared environment of twins in the womb, potentially leading to pregnancy complications.

It is a common misconception that two placentas means twins are dizygotic. But if monozygotic twins separate early enough, the arrangement of sacs and placentas in utero is indistinguishable from dizygotic twins.

Type	Description	Day
Dichorionic-Diamniotic	Normally, twins have two separate (di- being a numerical prefix for two) chorions and amniotic sacs, termed Dichorionic-Diamniotic or "DiDi". It occurs in almost all cases of dizygotic twins (except in very rare cases of fusion between their blastocysts^[35] ) and in 18–36%^[36] (or around 25%^[35]) of monozygotic (identical) twins. DiDi twins have the lowest mortality risk at about 9 percent, although that is still significantly higher than that of singletons.^[37]	Dichorionic-Diamniotic twins form when splitting takes place by the third day after fertilization.^[35]
Monochorionic-Diamniotic	Monochorionic twins share the same placenta. Monochorionic twins generally have two amniotic sacs (called Monochorionic-Diamniotic "MoDi"), which occurs in 60–70% of the pregnancies with monozygotic twins,^[36] and in 0.3% of all pregnancies.^[38] Monochorionic-Diamniotic twins are almost always monozygotic, with a few exceptions where the blastocysts have fused.^[35] Monochorionic twins share the same placenta, and thus have a risk of twin-to-twin transfusion syndrome.	Days 4-8
Monochorionic-Monoamniotic	Monochorionic twins share the same amnion in 1–2% of monozygotic twin pregnancies.^[36] Monoamniotic twins are always monozygotic.^[39] The survival rate for monoamniotic twins is somewhere between 50%^[39] to 60%.^[40] Monoamniotic twins, as with diamniotic monochorionic twins, have a risk of twin-to-twin transfusion syndrome. Also, the two umbilical cords have an increased chance of being tangled around the babies. Because of this, there is an increased chance that the newborns may be miscarried or suffer from cerebral palsy due to lack of oxygen.	Monoamniotic twins occur when the split takes place after the ninth day after fertilization.^[35]
Conjoined twins	When the division of the developing zygote into 2 embryos occurs, 99% of the time it is within 8 days of fertilization. Mortality is highest for conjoined twins due to the many complications resulting from shared organs.	If the division of the zygote occurs later than the 12 days then conjoined twins are usually the result.

Dichorionic-diamniotic twins at 8 weeks and 5 days since co-incubation as part of IVF. The twin at left in the image is shown in the sagittal plane with the head pointing towards upper left. The twin at right in the image is shown in the coronal plane with the head pointing rightwards.
Abdominal ultrasonography of monoamniotic twins at a gestational age of 15 weeks. There is no sign of any membrane between the fetuses. A coronal plane is shown of the twin at left, and a sagittal plane of parts of the upper thorax and head is shown of the twin at right.

Demographics

A 2006 study has found that insulin-like growth factor present in dairy products may increase the chance of dizygotic twinning. Specifically, the study found that vegan mothers (who exclude dairy from their diets) are one-fifth as likely to have twins as vegetarian or omnivore mothers, and concluded that "Genotypes favoring elevated IGF and diets including dairy products, especially in areas where growth hormone is given to cattle, appear to enhance the chances of multiple pregnancies due to ovarian stimulation."^[41]

From 1980 to 1997, the number of twin births in the United States rose 52%.^[42] This rise can at least partly be attributed to the increasing popularity of fertility drugs like Clomid and procedures such as IVF, which result in multiple births more frequently than unassisted fertilizations do. It may also be linked to the increase of growth hormones in food.^[41]

Ethnicity

A pair of female ere ibeji twin figures (early 20th-century) in the permanent collection of The Children's Museum of Indianapolis. Yoruba people have the highest twinning rate in the world.

About 1 in 90 human births (1.1%) results from a twin pregnancy.^[43] The rate of dizygotic twinning varies greatly among ethnic groups, ranging as high as about 45 per 1000 births (4.5%) for the Yoruba to 10% for Linha São Pedro, a tiny Brazilian settlement which belongs to the city of Cândido Godói.^[44] In Cândido Godói, one in five pregnancies has resulted in twins.^[45] The Argentine historian Jorge Camarasa has put forward a theory that experiments of the Nazi doctor Josef Mengele could be responsible for the high ratio of twins in the area.^[46] His theory was rejected by Brazilian scientists who had studied twins living in Linha São Pedro; they suggested genetic factors within that community as a more likely explanation.^[47] A high twinning rate has also been observed in other places of the world, including :

Igbo-Ora in Nigeria^[48]
Kodinji, located in Kerala, India^[49]
Mohammadpur Umri, located in Uttar Pradesh, India^[50]^[51]^[52]

The widespread use of fertility drugs causing hyperovulation (stimulated release of multiple eggs by the mother) has caused what some call an "epidemic of multiple births". In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births. Nevertheless, the rate of monozygotic twins remains at about 1 in 333 across the globe.

In a study on the maternity records of 5750 Hausa women living in the Savannah zone of Nigeria, there were 40 twins and 2 triplets per 1000 births. Twenty-six percent of twins were monozygotic. The incidence of multiple births, which was about five times higher than that observed in any western population, was significantly lower than that of other ethnic groups, who live in the hot and humid climate of the southern part of the country. The incidence of multiple births was related to maternal age but did not bear any association to the climate or prevalence of malaria.^[53]^[54]

Predisposing factors

The predisposing factors of monozygotic twinning are unknown.

Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:

She is of West African descent (especially Yoruba)
She is between the age of 30 and 40 years
She is greater than average height and weight
She has had several previous pregnancies.

Women undergoing certain fertility treatments may have a greater chance of dizygotic multiple births. In the United States it has been estimated that by 2011 36% of twin births resulted from conception by assisted reproductive technology.^[55]

The risk of twin birth can vary depending on what types of fertility treatments are used. With in vitro fertilisation (IVF), this is primarily due to the insertion of multiple embryos into the uterus. Ovarian hyperstimulation without IVF has a very high risk of multiple birth. Reversal of anovulation with clomifene (trade names including Clomid) has a relatively less but yet significant risk of multiple pregnancy.

Delivery interval

A 15-year German study^[56] of 8,220 vaginally delivered twins (that is, 4,110 pregnancies) in Hesse yielded a mean delivery time interval of 13.5 minutes.^[57] The delivery interval between the twins was measured as follows:

Within 15 minutes: 75.8%
16-30 minutes: 16.4%
31-45 minutes: 4.3%
46-60 minutes: 1.7%
Over 60 minutes: 1.8% (72 instances)

The study stated that the occurrence of complications "was found to be more likely with increasing twin-to-twin delivery time interval" and suggested that the interval be kept short, though it noted that the study did not examine causes of complications and did not control for factors such as the level of experience of the obstetrician, the wish of the women giving birth, or the "management strategies" of the procedure of delivering the second twin.

Complications during pregnancy

Vanishing twins

Researchers suspect that as many as 1 in 8 pregnancies start out as multiples, but only a single fetus is brought to full term, because the other has died very early in the pregnancy and has not been detected or recorded.^[58] Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. This is known as vanishing twin syndrome. Also, in an unknown proportion of cases, two zygotes may fuse soon after fertilization, resulting in a single chimeric embryo, and, later, fetus.

Chang and Eng Bunker, born in Siam (now Thailand) in 1811, were the origin of the term "Siamese twins".

Conjoined twins

Conjoined twins (or the commonly used term "siamese") are monozygotic twins whose bodies are joined together during pregnancy. This occurs when the zygote starts to split after day 12^[35] following fertilization and fails to separate completely. This condition occurs in about 1 in 50,000 human pregnancies. Most conjoined twins are now evaluated for surgery to attempt to separate them into separate functional bodies. The degree of difficulty rises if a vital organ or structure is shared between twins, such as the brain, heart or liver.

Chimerism

A chimera is an ordinary person or animal except that some of their parts actually came from their twin or from the mother. A chimera may arise either from monozygotic twin fetuses (where it would be impossible to detect), or from dizygotic fetuses, which can be identified by chromosomal comparisons from various parts of the body. The number of cells derived from each fetus can vary from one part of the body to another, and often leads to characteristic mosaicism skin coloration in human chimeras. A chimera may be intersex, composed of cells from a male twin and a female twin. In one case DNA tests determined that a woman, mystifyingly, was not the mother of two of her three children; the mother was found to be a chimera and those of her cells tested happened to derive from one of the fused twins, while two of the children were derived from eggs derived from cells from the other twin making up their mother.^[59]

Parasitic twins

Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. One fetus acts as a parasite towards the other. Sometimes the parasitic twin becomes an almost indistinguishable part of the other, and sometimes this needs to be treated medically.

Partial molar twins

A very rare type of parasitic twinning is one where a single viable twin is endangered when the other zygote becomes cancerous, or molar. This means that the molar zygote's cellular division continues unchecked, resulting in a cancerous growth that overtakes the viable fetus. Typically, this results when one twin has either triploidy or complete paternal uniparental disomy, resulting in little or no fetus and a cancerous, overgrown placenta, resembling a bunch of grapes.

Miscarried twin

Occasionally, a woman will suffer a miscarriage early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term. This occurrence is similar to the vanishing twin syndrome, but typically occurs later, as the twin is not reabsorbed.

Low birth weight

Twins typically suffer from the lower birth weights and greater likelihood of prematurity that is more commonly associated with the higher multiple pregnancies.

Twin-to-twin transfusion syndrome

Monozygotic twins who share a placenta can develop twin-to-twin transfusion syndrome. This condition means that blood from one twin is being diverted into the other twin. One twin, the 'donor' twin, is small and anemic, the other, the 'recipient' twin, is large and polycythemic. The lives of both twins are endangered by this condition.

Management of birth

For otherwise healthy twin pregnancies where both twins are head down a trial of vaginal delivery is recommended at between 37 and 38 weeks.^[60]^[61] Vaginal delivery in this case does not worsen the outcome for the infant as compared with Caesarean section.^[60] There is controversy on the best method of delivery were the first twin is head first and the second is not.^[60] When the first twin is not head down a C-section is often recommended.^[60] It is estimated that 75% of twin pregnancies in the United States were delivered by Cesarean section in 2008.^[62] In comparison, the rate of Cesarean section for all pregnancies in the general population varies between 14% and 40%.^[63]

Human twin studies

Twin studies are utilized in an attempt to determine how much of a particular trait is attributable to either genetics or environmental influence. These studies compare monozygotic and dizygotic twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from epigenetic and environmental influence. Twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been used widely in the exploration of human nature. Classical twin studies are now being supplemented with molecular genetic studies which identify individual genes.

Unusual twinnings

Among dizygotic twins, in rare cases, the eggs are fertilized at different times with two or more acts of sexual intercourse, either within one menstrual cycle (superfecundation) or, even more rarely, later on in the pregnancy (superfetation). This can lead to the possibility of a woman carrying fraternal twins with different fathers (that is, half-siblings). This phenomenon is known as heteropaternal superfecundation. One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins, whose parents were involved in paternity suits, was approximately 2.4%; see the references section, below, for more details.

Dizygotic twins from biracial couples can sometimes be mixed twins, which exhibit differing ethnic and racial features. One such pairing was born in Germany in 2008 to a white father from Germany and a black mother from Ghana.^[64]^[65]

Heterotopic pregnancy is an exceedingly rare type of dizygotic twinning in which one twin implants in the uterus as normal and the other remains in the fallopian tube as an ectopic pregnancy. Ectopic pregnancies must be resolved because they can be life-threatening to the mother. However, in most cases, the intrauterine pregnancy can be salvaged.

Among monozygotic twins, in extremely rare cases, twins have been born with different sexes (one male, one female).^[66] When monozygotic twins are born with different sexes it is because of chromosomal defects. There can be monozygotic boy/girl twins if the sex gene of the embryo has an extra x chromosome (the fertilized egg would be an xxy) then when the egg splits, one can have xx (girl) genes and one can have xy (boy) genes. This is rare, but possible. Records show there are only 10 known cases of these type twins. The probability of this is so small that multiples having different sexes is universally accepted as a sound basis for in utero clinical determination that the multiples are not monozygotic. Another abnormality that can result in monozygotic twins of different genders is if the egg is fertilized by a male sperm but during cell division only the X chromosome is duplicated. This results in one normal male (XY) and one female with Turner syndrome (45,X).^[67] In these cases, although the twins did form from the same fertilized egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes.

Semi-identical twins

Monozygotic twins can develop differently, due to different genes being activated.^[68] More unusual are "semi-identical twins". These "half-identical twins" are hypothesized to occur when an unfertilized egg cleaves into two identical attached ova and which are viable for fertilization. Both cloned ova are then fertilized by different sperm and the coalesced eggs undergo further cell duplications developing as a chimeric blastomere. If this blastomere then undergoes a twinning event, two embryos will be formed, each of which have different paternal genes and identical maternal genes.

This results in a set of twins with identical genes from the mother's side, but different genes from the father's side. Cells in each fetus carry genes from either sperm, resulting in chimeras. This form had been speculated until only recently being recorded in western medicine.^[69]^[70]^[71]

Twin calves of the Hereford breed in Miles City, Montana

Mirror image twins

Mirror image twins result when a fertilized egg splits later in the embryonic stage than normal timing, around day 9-12. This type of twinning could exhibit characteristics with reversed asymmetry, such as opposite dominant handedness, dental structure, or even organs (situs inversus).^[72] If the split occurs later than this time period, the twins risk being conjoined. There is no DNA-based zygosity test that can determine if twins are indeed mirror image.^[73]

Language development

There have been many studies highlighting the development of language in twins compared to single-born children. These studies have converged on the notion that there is a greater rate of delay in language development in twins compared to their single-born counterparts.^[74] The reasons for this phenomenon are still in question; however, idioglossia was thought to be the major cause.^[75] Idioglossia is defined as a private language that is invented usually by young children, specifically twins. The increased focused communication between two twins may isolate them from the social environment surrounding them. Idioglossia has been found to be a rare occurrence and the attention of scientists has shifted away from this idea. Current research is looking into the impacts of a richer social environment for these twins to stimulate their development of language.^[76]

Other animals

Twins are common in many animal species, including cats, sheep, ferrets, giant pandas, dogs, deer, marmosets, and tamarins. The incidence of twinning among cattle is about 1–4%, and research is under way to improve the odds of twinning, which can be more profitable for the breeder if complications can be sidestepped or managed. A female calf that is the twin of a bull becomes partially masculinized and is known as a freemartin.

References

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^ Martin, Joyce A.; Hamilton, Brady E.; Osterman, Michelle J.K. "Three Decades of Twin Births in the United States, 1980–2009" [1], National Center for Health Statistics Data Brief, No. 80, January 2012
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^ Cheng, Maria; Associated Press (2007-07-03). "Study: Twins form after embryo collapses". USA Today. Retrieved 2008-09-30.
^ ^a ^b April Holladay (2001-05-09). "What triggers twinning?". WonderQuest. Retrieved 2007-03-22.
^ Illmensee K, Levanduski M, Vidali A, Husami N, Goudas VT (February 2009). "Human embryo twinning with applications in reproductive medicine". Fertil. Steril. 93 (2): 423–7. doi:10.1016/j.fertnstert.2008.12.098. PMID 19217091.
^ Oleszczuk, Jaroslaw J.; Donald M. Keith, Louis G. Keith and William F. Rayburn (November 1999). "Projections of population-based twinning rates through the year 2100". The Journal of Reproductive Medicine 44 (11): 913–921. PMID 10589400. Retrieved 2008-10-02.
^ "Time-lapse recordings reveal why IVF embryos are more likely to develop into twins. Researchers believe the laboratory culture could be the cause" (Press release). European Society of Human Reproduction and Embryology. 2007-07-02. Retrieved 2008-09-30.
^ Patwari, P; Lee, RT (2008-08-01). "Mechanical control of tissue morphogenesis.". Circulation research 103 (3): 234–43. doi:10.1161/CIRCRESAHA.108.175331. PMID 18669930.
^ Edwards, JH; T Dent and J Kahn (June 1966). "Monozygotic twins of different sex". Journal of Medical Genetics 3 (2): 117–123. doi:10.1136/jmg.3.2.117. PMC 1012913. PMID 6007033.
^ Machin, GA (January 1996). "Some causes of genotypic and phenotypic discordance in monozygotic twin pairs". American Journal of Medical Genetics 61 (3): 216–228. doi:10.1002/(SICI)1096-8628(19960122)61:3<216::AID-AJMG5>3.0.CO;2-S. PMID 8741866.
^ "Prenatal diagnosis of heterokaryotypic mosaic twins discordant for fetal sex.". Prenat Diagn 20 (12): 999–1003. Dec 2000. doi:10.1002/1097-0223(200012)20:12<999::aid-pd948>3.0.co;2-e. PMID 11113914.
^ Li, Rui et al. (2014). "Somatic point mutations occurring early in development: a monozygotic twin study". Journal of Medical Genetics 51: 28–44. Retrieved 2015-02-15.
^ Fraga, Mario F.; Esteban Ballestar, Maria F. Paz, Santiago Ropero, Fernando Setien, Maria L. Ballestar†, Damia Heine-Suñer, Juan C. Cigudosa, Miguel Urioste, Javier Benitez, Manuel Boix-Chornet, Abel Sanchez-Aguilera, Charlotte Ling, Emma Carlsson, Pernille Poulsen, Allan Vaag, Zarko Stephan, Tim D. Spector, Yue-Zhong Wu, Christoph Plass and Manel Esteller (July 2005). "Epigenetic differences arise during the lifetime of monozygotic twins". Proc. Natl. Acad. Sci. U.S.A. 102 (30): 10604–9. doi:10.1073/pnas.0500398102. PMC 1174919. PMID 16009939.
^ Segal, Nancy L. (1999). Entwined lives: twins and what they tell us about human behavior. New York: Dutton. ISBN 0-525-94465-6. OCLC 40396458. ^{[page needed]}
^ Plomin, Robert (2001). Behavioral genetics. New York: Worth Pubs. ISBN 0-7167-5159-3. OCLC 43894450. ^{[page needed]}
^ Mandler,G. (2001) Apart from genetics: What makes monozygotic twins similar? Journal of Mind and Behavior,22,147-159.
^ Bieber, Frederick R.; Walter E. Nance; Cynthia C. Morton; Judith A. Brown; Fay O. Redwine; Robert L. Jordan and T. Mohanakumar (1981-08-14). "Genetic Studies of an Acardiac Monster: Evidence of Polar Body Twinning in Man". Science (American Association for the Advancement of Science) 213 (4509): 775–777. doi:10.1126/science.7196086. JSTOR 1686613. PMID 7196086. Non-identical Monozygotic Twins Lay summary – DevBio: A Companion to Developmental Biology (2006).
^ Golubovsky, M.D. (February 2003). "Postzygotic diploidization of triploids as a source of unusual cases of mosaicism, chimerism and twinning". Human Reproduction (Oxford Journals) 18 (2): 236–242. doi:10.1093/humrep/deg060. PMID 12571155. Retrieved 2008-02-29. 'Semi-identical' twins discovered Lay summary – Nature News (2007-03-26).
^ Souter, Vivienne L.; Melissa A. Parisi, Dale R. Nyholt, Raj P. Kapur, Anjali K. Henders, Kent E. Opheim, Daniel F. Gunther, Michael E. Mitchell, Ian A. Glass and Grant W. Montgomery (April 2007). "A case of true hermaphroditism reveals an unusual mechanism of twinning". Human Genetics (Berlin / Heidelberg: Springer) 121 (2): 179–185. doi:10.1007/s00439-006-0279-x. ISSN 1432-1203. PMID 17165045. Retrieved 2008-02-29. Semi-identical twins discovered Lay summary – BBC News (2007-02-27).
^ ^a ^b ^c ^d ^e ^f Shulman, Lee S.; Vugt, John M. G. van (2006). Prenatal medicine. Washington, DC: Taylor & Francis. p. Page 447. ISBN 0-8247-2844-0.
^ ^a ^b ^c Curran, Mark (2005-11-02). "Twinning". Focus Information Technology. Retrieved 2008-10-10.
^ Benirschke, Kurt (2004). "Multiple Gestation". In Robert Resnik, Robert K. Creasy and Jay D. Iams. Maternal-Fetal Medicine: Principles and Practice (5th ed.). Philadelphia: W. B. Saunders Company. pp. 55–62. ISBN 0-7216-0004-2.
^ Cordero L, Franco A, Joy SD, O'shaughnessy RW (December 2005). "Monochorionic diamniotic infants without twin-to-twin transfusion syndrome". J Perinatol 25 (12): 753–8. doi:10.1038/sj.jp.7211405. PMID 16281049.
^ ^a ^b Pregnancy-Info -- > Monoamniotic Twins Retrieved on July 9, 2009
^ MoMo Twins; Monochorionic Monoamniotic Twins By Pamela Prindle Fierro, About.com. Retrieved on July 9, 2009
^ ^a ^b Steinman, Gary (May 2006). "Mechanisms of twinning: VII. Effect of diet and heredity on the human twinning rate". J Reprod Med 51 (5): 405–410. PMID 16779988. Retrieved 2008-09-29.
^ Martin, Joyce A; Melissa M. Park (1999-09-14). "Trends in Twin and Triplet Births: 1980–97" (PDF). National Vital Statistics Reports (National Center for Health Statistics) 47 (24): 1–17. Retrieved 2008-09-29.
^ Asch, Richard H.; John Studd (1995). Progress in Reproductive Medicine Volume II. Informa. ISBN 1-85070-574-7. ISSN 1358-8702. OCLC 36287045. ^{[page needed]}
^ Matte, U; MG Le Roux, B Bénichou, JP Moisan and R Giugliani (1996). "Study on possible increase in twinning rate at a small village in south Brazil". Acta Genet Med Gemellol (Roma) 45 (4): 431–437. PMID 9181177.
^ Nick Evans (21 January 2009). "Nazi Angel of death Josef Mengel created twin town in Brazil". The Telegraph.
^ Caroline Iggulden (23 January 2009). "The twins from Brazil". The Sun.
^ Linda Geddes: Nazi 'Angel of Death' not responsible for town of twins New Scientist online, 27 January 2009
^ "The Land Of Twins". BBC World News. 7 June 2001. Retrieved 2009-08-06.
^ "Seeing double: the village in deepest Kerala where twins have taken over". The Independent. 12 May 2009. Retrieved 2009-08-06.
^ "A village of twins". http://timesofindia.indiatimes.com. Bennett, Coleman & Co. Ltd. Retrieved 23 October 2014.
^ "Twin Capital Of The World". http://www.littleindia.com. 4Skylark.com. Retrieved 23 October 2014.
^ "Mohammadpur Umri , THE VILLAGE OF TWINS !!". http://himsamachar.com. Business Directory Plugin. Retrieved 23 October 2014.
^ Rehan, N; DS Tafida (November 1980). "Multiple births in Hausa women". Br J Obstet Gynaecol 87 (11): 997–1004. doi:10.1111/j.1471-0528.1980.tb04464.x. PMID 7437372.
^ Yoruba Customs and Beliefs Pertaining to Twins. Randafricanart.com. Retrieved on 2013-08-10.
^ Kulkarni, A. D.; Jamieson, D. J.; Jones Jr, H. W.; Kissin, D. M.; Gallo, M. F.; MacAluso, M.; Adashi, E. Y. (2013). "Fertility Treatments and Multiple Births in the United States". New England Journal of Medicine 369 (23): 2218–2225. doi:10.1056/NEJMoa1301467. PMID 24304051. edit
^ "Twin-to-twin delivery time interval: influencing factors and effect on short-term outcome of the second twin"; Werner Stein, Björn Misselwitz & Stefan Schmidt; Acta Obstetricia et Gynecologica Scandinavica, 87(3):346-353, 2008.
^ There were 836,104 deliveries of babies in Hesse over the 15-year study period, including 11,740 twin pregnancies, of which only 4,110 met the inclusion criteria and hence were examined in the study. The excluded twin pregnancies were in cases of (1) delivery before 34 weeks of gestation; (2) when the first twin was delivered by caesarean section; (3) when either of the twins had died in the womb before the onset of labor; and (4) when the pregnancy had been complicated by fetal malformations or Twin-to-twin transfusion syndrome.
^ Gedda, Luigi (1995). "The role of research in twin medicine". In Louis G. Keith, Emile Papiernik, Donald M. Keith and Barbara Luke. Multiple Pregnancy: Epidemiology, Gestation & Perinatal Outcome. Taylor and Francis. pp. 4. ISBN 1-85070-666-2.
^ New scientist magazine: The stranger within. 15 November 2003, p34
^ ^a ^b ^c ^d Biswas, A; Su, LL; Mattar, C (Apr 2013). "Caesarean section for preterm birth and, breech presentation and twin pregnancies.". Best practice & research. Clinical obstetrics & gynaecology 27 (2): 209–19. doi:10.1016/j.bpobgyn.2012.09.002. PMID 23062593.
^ National Collaborating Centre for Women’s and Children’s Health (November 2011). "Caesarean section" (PDF). NICE. Royal College of Obstetricians and Gynaecologist. pp. 70–103. Retrieved 12 June 2014.
^ Lee HC, Gould JB, Boscardin WJ, El-Sayed YY, Blumenfeld YJ (2011). "Trends in cesarean delivery for twin births in the United States: 1995-2008". Obstet Gynecol 118 (5): 1095–101. doi:10.1097/AOG.0b013e3182318651. PMC 3202294. PMID 22015878.
^ Women can choose Caesarean birth - BBC article by James Gallagher. 2011.
^ Lovell, Tammy (2008-07-17). "Pictured: Proud parents show off their million-to-one black and white twins". Daily Mail (Associated Newspapers Ltd). Retrieved 2008-10-02.
^ Moorhead, Joanna (2011-09-24). "Black and white twins". The Guardian. Retrieved 2012-04-07.
^ Schmidt, R; EH Sobel, HM Nitowsky, H Dar and FH Allen Jr (February 1976). "Monozygotic twins discordant for sex". Journal of Medical Genetics 13 (1): 64–68. doi:10.1136/jmg.13.1.64. PMC 1013354. PMID 944787. Retrieved 2008-09-29.
^ http://www.twinsmagazine.com/images/zygosityMA03.pdf
^ Gilbert, Scott F. (2006). "Non-identical Monozygotic Twins". Developmental biology. Sunderland, Mass: Sinauer Associates. ISBN 0-87893-250-X. OCLC 172964621. Retrieved 2008-09-30.
^ Souter, Vivienne L.; Melissa A. Parisi, Dale R. Nyholt, Raj P. Kapur, Anjali K. Henders, Kent E. Opheim, Daniel F. Gunther, Michael E. Mitchell, Ian A. Glass and Grant W. Montgomery (April 2007). "A case of true hermaphroditism reveals an unusual mechanism of twinning". Hum. Genet. 121 (2): 179–85. doi:10.1007/s00439-006-0279-x. PMID 17165045.
^ LiveScience Staff (2007-03-26). "Rare Semi-Identical Twins Discovered". Imaginova. Retrieved 2008-10-01.
^ Whitfield, John (2007). "'Semi-identical' twins discovered". Nature. doi:10.1038/news070326-1.
^ Gedda L, Sciacca A, Brenci G et al. (1984). "Situs viscerum specularis in monozygotic twins". Acta Genet Med Gemellol (Roma) 33 (1): 81–5. PMID 6540028.
^ Twins genetics, details of environmental impacts. Proactivegenetics.com. Retrieved on 2013-08-10.
^ Thorpe, K (June 2006). "Twin children's language development". Early Human Development 82 (6): 387–395. doi:10.1016/j.earlhumdev.2006.03.012.
^ Usher, L (13 October 2000). "Idioglossia". Times Literary Supplement: 24.
^ Hayashi, C (2007). "Relationship between Twin Language, Twins' Bond, and Social Competence". Twin research and human genetics.

Foundations

Tamba, Twins and Multiple Births Association

UpToDate Contents

全文を閲覧するには購読必要です。 To read the full text you will need to subscribe.

1. Twin pregnancy: Prenatal issues
2. 一絨毛膜性双胎妊娠 monoamniotic twin pregnancy
3. 双胎妊娠：陣痛および分娩 twin pregnancy labor and delivery
4. 多胎妊娠の新生児の合併症、転帰、およびマネージメント neonatal complications outcome and management of multiple births
5. 双胎間輸血症候群：病因および診断 twin twin transfusion syndrome pathogenesis and diagnosis

English Journal

The Heritability of Breast Cancer among women in the Nordic Twin Study of Cancer.

Möller S1, Mucci LA2, Harris JR3, Scheike T4, Holst K4, Halekoh U5, Adami HO6, Czene K6, Christensen K7, Holm NV8, Pukkala E9, Skytthe A10, Kaprio J11, Hjelmborg JB12.
Cancer epidemiology, biomarkers & prevention : a publication of the American Association for Cancer Research, cosponsored by the American Society of Preventive Oncology.Cancer Epidemiol Biomarkers Prev.2015 Nov 10. pii: cebp.0913.2015. [Epub ahead of print]
Background Family history is an established risk factor for breast cancer. Although some important genetic factors have been identified, the extent to which familial risk can be attributed to genetic factors versus common environment remains unclear. Methods We estimated the familial concordance and
PMID 26554920

Anxiety is associated with increased risk of dementia in older Swedish twins.

Petkus AJ1, Reynolds CA2, Wetherell JL3, Kremen WS4, Pedersen NL5, Gatz M5.
Alzheimer's & dementia : the journal of the Alzheimer's Association.Alzheimers Dement.2015 Nov 5. pii: S1552-5260(15)02914-3. doi: 10.1016/j.jalz.2015.09.008. [Epub ahead of print]
INTRODUCTION: We asked whether anxiety is associated with prospective risk of dementia, and the extent to which genetic influences mediate this association.METHODS: Nondemented twins (n = 1082) from the Swedish Adoption Twin Study of Aging completed an assessment of anxiety symptoms in 1984 and we
PMID 26549599

A review of the mechanisms and evidence for typical and atypical twinning.

Mcnamara HC1, Kane SC2, Craig JM3, Short RV4, Umstad MP5.
American journal of obstetrics and gynecology.Am J Obstet Gynecol.2015 Nov 5. pii: S0002-9378(15)02235-8. doi: 10.1016/j.ajog.2015.10.930. [Epub ahead of print]
The mechanisms responsible for twinning and disorders of twin gestations have been the subject of considerable interest by physicians and scientists, and cases of atypical twinning have called for a reexamination of the fundamental theories invoked to explain twin gestations. This article presents a
PMID 26548710

Japanese Journal

予防医学における双生児研究の重要性 : エピジェネティクス時代の遺伝-環境因子の交絡研究

早川和生
日本衞生學雜誌 66(1), 29-30, 2011-01-15
The importance of twin research in the field of preventive medicine is described from the viewpoint of gene-environment interaction. The recent advancements in twin research in Japan and other countri …
NAID 10029363631

中高生における二次元レジリエンス要因尺度（BRS）の妥当性 : ――双生児法による検討

平野真理
パーソナリティ研究 20(1), 50-52, 2011
… In Study 2, the BRS was administered to 41 monozygotic (MZ) and 15 dizygotic (DZ) twin pairs. … The intraclass correlation coefficient (ICC) of the innate resilience factor was significant for MZ but not DZ twins, thus making the BRS scored attributable to independent genetic factors. …
NAID 130001478428

Related Pictures

★リンクテーブル★

リンク元	「二卵性双胎」
関連記事	「twin」「dizygotic」

「二卵性双胎」

　　[★]

英: dizygotic twin dizygotic twins, biovular twins
ラ: gemini dizygotici、biovulares dizygotici
関: 双胎

「twin」

　　[★]

n.

双子、双生児

関: geminal、gemini、geminus

「dizygotic」

　　[★]

二卵性の

関: binovular、fraternal

[MedicineNet-1] MedicineNet > Definition of Twin Last Editorial Review: 6/19/2000

[2] Michael R. Cummings "Human Heredity Principles and issues" pg 104

[3] "Twins, Triplets, Multiple Births: MedlinePlus". Nlm.nih.gov. Retrieved 2013-09-16.

[4] Martin, Joyce A.; Hamilton, Brady E.; Osterman, Michelle J.K. "Three Decades of Twin Births in the United States, 1980–2009" [1], National Center for Health Statistics Data Brief, No. 80, January 2012

[5] Zach, Terence; Arun K Pramanik and Susannah P Ford (2007-10-02). "Multiple Births". WebMD. Retrieved 2008-09-29.

[6] "Genetics or yams in the Land of Twins?". Independent Online. 2007-11-12. Retrieved 2008-09-29.

[7] "The Land of Twins". BBC World Service. 2001-06-07. Retrieved 2008-09-29.

[Keith-8] O. Bomsel-Helmreich; W. Al Mufti (1995). "The mechanism of monozygosity and double ovulation". In Louis G. Keith, Emile Papierik, Donald M. Keith and Barbara Luke. Multiple Pregnancy: Epidemiology, Gestation & Perinatal Outcome. Taylor and Francis. p. 34. ISBN 1-85070-666-2.

[9] "What’s in a yam? Clues to fertility, a student discovers". 1999. Retrieved 2008-09-29.

[Smits-10] Smits, Jeroen; Christiaan Monden (2011). Newell, Marie-Louise, ed. "Twinning across the Developing World". PLoS ONE (Public Library of Science) 6 (9): e25239. doi:10.1371/journal.pone.0025239. PMC 3182188. PMID 21980404.

[11] Elliott, JP (December 2008). "Preterm labor in twins and high-order multiples". Clinical Perinatology 34 (4): 599–609. doi:10.1016/j.clp.2007.10.004. PMID 18063108. Unlike singleton gestation where identification of patients at risk for PTL is often difficult, every multiple gestation is at risk for PTL, so all patients can be managed as being at risk.

[twinheredity-12] "Probability of having twins hereditary?". Go Ask Alice!. Retrieved 15 March 2012.

[13] Fitch, Karen. "Ask a Geneticist". Understanding Genetics. Stanford School of Medicine. Retrieved 15 March 2012.

[multi-14] "Identical and Fraternal Twins – Determining Zygosity in Twins". Multiples.about.com. 2013-07-16. Retrieved 2013-09-16.

[15] "United States: People". The World Factbook. Central Intelligence Agency. 2008-09-04. Retrieved 2008-10-02.

[16] "Italy: People". The World Factbook. Central Intelligence Agency. 2008-09-04. Retrieved 2008-10-02.

[Bortolus-17] Bortolus, Renata; Fabio Parazzini; Liliane Chatenoud; Guido Benzi; Massimiliano Maria Bianchi; Alberto Marini (1999). "The epidemiology of multiple births". Human Reproduction Update (European Society of Human Reproduction and Embryology) 5 (2): 179–187. doi:10.1093/humupd/5.2.179. ISSN 1355-4786. PMID 10336022.

[18] Cheng, Maria; Associated Press (2007-07-03). "Study: Twins form after embryo collapses". USA Today. Retrieved 2008-09-30.

[wonderquest-19] April Holladay (2001-05-09). "What triggers twinning?". WonderQuest. Retrieved 2007-03-22.

[20] Illmensee K, Levanduski M, Vidali A, Husami N, Goudas VT (February 2009). "Human embryo twinning with applications in reproductive medicine". Fertil. Steril. 93 (2): 423–7. doi:10.1016/j.fertnstert.2008.12.098. PMID 19217091.

[21] Oleszczuk, Jaroslaw J.; Donald M. Keith, Louis G. Keith and William F. Rayburn (November 1999). "Projections of population-based twinning rates through the year 2100". The Journal of Reproductive Medicine 44 (11): 913–921. PMID 10589400. Retrieved 2008-10-02.

[22] "Time-lapse recordings reveal why IVF embryos are more likely to develop into twins. Researchers believe the laboratory culture could be the cause" (Press release). European Society of Human Reproduction and Embryology. 2007-07-02. Retrieved 2008-09-30.

[23] Patwari, P; Lee, RT (2008-08-01). "Mechanical control of tissue morphogenesis.". Circulation research 103 (3): 234–43. doi:10.1161/CIRCRESAHA.108.175331. PMID 18669930.

[24] Edwards, JH; T Dent and J Kahn (June 1966). "Monozygotic twins of different sex". Journal of Medical Genetics 3 (2): 117–123. doi:10.1136/jmg.3.2.117. PMC 1012913. PMID 6007033.

[25] Machin, GA (January 1996). "Some causes of genotypic and phenotypic discordance in monozygotic twin pairs". American Journal of Medical Genetics 61 (3): 216–228. doi:10.1002/(SICI)1096-8628(19960122)61:3<216::AID-AJMG5>3.0.CO;2-S. PMID 8741866.

[Schmid2000-26] "Prenatal diagnosis of heterokaryotypic mosaic twins discordant for fetal sex.". Prenat Diagn 20 (12): 999–1003. Dec 2000. doi:10.1002/1097-0223(200012)20:12<999::aid-pd948>3.0.co;2-e. PMID 11113914.

[J_Med_Genet-27] Li, Rui et al. (2014). "Somatic point mutations occurring early in development: a monozygotic twin study". Journal of Medical Genetics 51: 28–44. Retrieved 2015-02-15.

[28] Fraga, Mario F.; Esteban Ballestar, Maria F. Paz, Santiago Ropero, Fernando Setien, Maria L. Ballestar†, Damia Heine-Suñer, Juan C. Cigudosa, Miguel Urioste, Javier Benitez, Manuel Boix-Chornet, Abel Sanchez-Aguilera, Charlotte Ling, Emma Carlsson, Pernille Poulsen, Allan Vaag, Zarko Stephan, Tim D. Spector, Yue-Zhong Wu, Christoph Plass and Manel Esteller (July 2005). "Epigenetic differences arise during the lifetime of monozygotic twins". Proc. Natl. Acad. Sci. U.S.A. 102 (30): 10604–9. doi:10.1073/pnas.0500398102. PMC 1174919. PMID 16009939.

[29] Segal, Nancy L. (1999). Entwined lives: twins and what they tell us about human behavior. New York: Dutton. ISBN 0-525-94465-6. OCLC 40396458. ^{[page needed]}

[30] Plomin, Robert (2001). Behavioral genetics. New York: Worth Pubs. ISBN 0-7167-5159-3. OCLC 43894450. ^{[page needed]}

[31] Mandler,G. (2001) Apart from genetics: What makes monozygotic twins similar? Journal of Mind and Behavior,22,147-159.

[32] Bieber, Frederick R.; Walter E. Nance; Cynthia C. Morton; Judith A. Brown; Fay O. Redwine; Robert L. Jordan and T. Mohanakumar (1981-08-14). "Genetic Studies of an Acardiac Monster: Evidence of Polar Body Twinning in Man". Science (American Association for the Advancement of Science) 213 (4509): 775–777. doi:10.1126/science.7196086. JSTOR 1686613. PMID 7196086. Non-identical Monozygotic Twins Lay summary – DevBio: A Companion to Developmental Biology (2006).

[33] Golubovsky, M.D. (February 2003). "Postzygotic diploidization of triploids as a source of unusual cases of mosaicism, chimerism and twinning". Human Reproduction (Oxford Journals) 18 (2): 236–242. doi:10.1093/humrep/deg060. PMID 12571155. Retrieved 2008-02-29. 'Semi-identical' twins discovered Lay summary – Nature News (2007-03-26).

[34] Souter, Vivienne L.; Melissa A. Parisi, Dale R. Nyholt, Raj P. Kapur, Anjali K. Henders, Kent E. Opheim, Daniel F. Gunther, Michael E. Mitchell, Ian A. Glass and Grant W. Montgomery (April 2007). "A case of true hermaphroditism reveals an unusual mechanism of twinning". Human Genetics (Berlin / Heidelberg: Springer) 121 (2): 179–185. doi:10.1007/s00439-006-0279-x. ISSN 1432-1203. PMID 17165045. Retrieved 2008-02-29. Semi-identical twins discovered Lay summary – BBC News (2007-02-27).

[Shulman-35] ^ ^a ^b ^c ^d ^e ^f Shulman, Lee S.; Vugt, John M. G. van (2006). Prenatal medicine. Washington, DC: Taylor & Francis. p. Page 447. ISBN 0-8247-2844-0.

[splitstats-36] Curran, Mark (2005-11-02). "Twinning". Focus Information Technology. Retrieved 2008-10-10.

[37] Benirschke, Kurt (2004). "Multiple Gestation". In Robert Resnik, Robert K. Creasy and Jay D. Iams. Maternal-Fetal Medicine: Principles and Practice (5th ed.). Philadelphia: W. B. Saunders Company. pp. 55–62. ISBN 0-7216-0004-2.

[cordero-38] Cordero L, Franco A, Joy SD, O'shaughnessy RW (December 2005). "Monochorionic diamniotic infants without twin-to-twin transfusion syndrome". J Perinatol 25 (12): 753–8. doi:10.1038/sj.jp.7211405. PMID 16281049.

[pregnancy-info-39] Pregnancy-Info -- > Monoamniotic Twins Retrieved on July 9, 2009

[about-40] MoMo Twins; Monochorionic Monoamniotic Twins By Pamela Prindle Fierro, About.com. Retrieved on July 9, 2009

[vegan-41] Steinman, Gary (May 2006). "Mechanisms of twinning: VII. Effect of diet and heredity on the human twinning rate". J Reprod Med 51 (5): 405–410. PMID 16779988. Retrieved 2008-09-29.

[42] Martin, Joyce A; Melissa M. Park (1999-09-14). "Trends in Twin and Triplet Births: 1980–97" (PDF). National Vital Statistics Reports (National Center for Health Statistics) 47 (24): 1–17. Retrieved 2008-09-29.

[asch-43] Asch, Richard H.; John Studd (1995). Progress in Reproductive Medicine Volume II. Informa. ISBN 1-85070-574-7. ISSN 1358-8702. OCLC 36287045. ^{[page needed]}

[44] Matte, U; MG Le Roux, B Bénichou, JP Moisan and R Giugliani (1996). "Study on possible increase in twinning rate at a small village in south Brazil". Acta Genet Med Gemellol (Roma) 45 (4): 431–437. PMID 9181177.

[45] Nick Evans (21 January 2009). "Nazi Angel of death Josef Mengel created twin town in Brazil". The Telegraph.

[46] Caroline Iggulden (23 January 2009). "The twins from Brazil". The Sun.

[47] Linda Geddes: Nazi 'Angel of Death' not responsible for town of twins New Scientist online, 27 January 2009

[48] "The Land Of Twins". BBC World News. 7 June 2001. Retrieved 2009-08-06.

[49] "Seeing double: the village in deepest Kerala where twins have taken over". The Independent. 12 May 2009. Retrieved 2009-08-06.

[50] "A village of twins". http://timesofindia.indiatimes.com. Bennett, Coleman & Co. Ltd. Retrieved 23 October 2014.

[51] "Twin Capital Of The World". http://www.littleindia.com. 4Skylark.com. Retrieved 23 October 2014.

[52] "Mohammadpur Umri , THE VILLAGE OF TWINS !!". http://himsamachar.com. Business Directory Plugin. Retrieved 23 October 2014.

[hausa-53] Rehan, N; DS Tafida (November 1980). "Multiple births in Hausa women". Br J Obstet Gynaecol 87 (11): 997–1004. doi:10.1111/j.1471-0528.1980.tb04464.x. PMID 7437372.

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dizygotic twins