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the placenta and fetal membranes that are expelled from the uterus after the baby is born

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後産(あとざん)(出産後,胎盤などが出ること)

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/07/26 15:21:47」(JST)

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Placental expulsion (also called afterbirth) occurs when the placenta comes out of the birth canal after childbirth. The period from just after the baby is expelled until just after the placenta is expelled is called the third stage of labor.

Physiology

It begins as a physiological separation from the wall of the uterus. The placenta is usually expelled within 15–30 minutes of the baby being born.

Maternal blood loss is limited by contraction of the uterus following birth of the placenta. Normal blood loss is less than 600 mL.

Management

The third stage of labor can be managed actively, or it could be managed expectantly (also known as physiological management or passive management), the latter allowing the placenta to be expelled without medical assistance.

Active management routinely involves clamping of the umbilical cord, often within seconds or minutes of birth. It may also involve giving oxytocin via intramuscular injection, followed by cord traction to assist in delivering the placenta. The oxytocic agents augment uterine muscular contraction and the cord traction assists with rapid birth of the placenta. However, premature cord traction can pull the placenta before it has naturally detached from the uterine wall, resulting in hemorrhage.

A Cochrane database study^[1] suggests that blood loss and the risk of postpartum bleeding will be reduced in women offered active management of the third stage of labour. However, the use of ergometrine for active management was associated with nausea or vomiting and hypertension, and controlled cord traction requires the immediate clamping of the umbilical cord.

A summary^[2] of the Cochrane study came to the results that active management of the third stage of labour, consisting of controlled cord traction, early cord clamping plus drainage, and a prophylactic oxytocic agent, reduced postpartum haemorrhage by 500 or 1000 mL or greater, as well as related morbidities including mean blood loss, incidences of postpartum haemoglobin becoming less than 9 g/dL, blood transfusion, need for supplemental iron postpartum, and length of third stage of labour. Although active management increased adverse effects such as nausea, vomiting, and headache, women were less likely to be dissatisfied.^[2]

Breastfeeding soon after birth and massaging of the top of the uterus (the fundus) causes uterine contractions that encourage birth of the placenta.

Although uncommon, in some cultures the placenta is kept and consumed by the mother over the weeks following the birth. This practice is termed placentophagy.

Retained placenta

A retained placenta is a placenta that doesn't undergo expulsion within a normal time limit. Risks of retained placenta include hemorrhage and infection. If the placenta fails to deliver in 30 minutes in a hospital environment, manual extraction may be required if heavy ongoing bleeding occurs, and very rarely a curettage is necessary to ensure that no remnants of the placenta remain (in rare conditions with very adherent placenta, placenta accreta). However, in birth centers and attended home birth environments, it is common for licensed care providers to wait for the placenta's birth up to 2 hours in some instances.

Non-humans

In most mammalian species, the mother bites through the cord and consumes the placenta, primarily for the benefit of prostaglandin on the uterus after birth. This is known as placentophagy. However, it has been observed in zoology that chimpanzees apply themselves to nurturing their offspring, and keep the fetus, cord, and placenta intact until the cord dries and detaches the next day.

The placenta exists in most mammals and some reptiles. It is probably polyphyletic, having arisen separately in evolution rather than being inherited from one distant common ancestor.

Studies on pigs indicate that the duration of placenta expulsion increases significantly with increased duration of farrowing.^[3]

References

^ Prendiville, W. J.; Elbourne, D.; McDonald, S. J.; Begley, C. M. (2000). Active versus expectant management in the third stage of labour. In Begley, Cecily M. "Cochrane Database of Systematic Reviews". Cochrane Database of Systematic Reviews (3). doi:10.1002/14651858.CD000007. edit
^ ^a ^b BMJ summary of the Cochrane group metanalysis, at Postpartum Hemorrhage: prevention by David Chelmow.
^ Rens, B.; Van Der Lende, T. (2004). "Parturition in gilts: duration of farrowing, birth intervals and placenta expulsion in relation to maternal, piglet and placental traits". Theriogenology 62 (1–2): 331–352. doi:10.1016/j.theriogenology.2003.10.008. PMID 15159125. edit

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