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== Placental Expulsion ==
 
== Placental Expulsion ==
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* In most species, expulsion of foetal membranes quickly follows expulsion of the foetus.   
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In most species, expulsion of foetal membranes quickly follows expulsion of the foetus.   
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* After the birth of the young, regular abdominal contractions largely cease.
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* Myometrial contractions persist.
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** Decreased amplitude, but become more frequent and less regular.
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** Important for dehiscence and expulsion of foetal membranes.
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* Waves of contractions from uterus to the cervix persist.
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** Act in a peristaltic fashion in the cow and sow in the reverse direction.
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=== Placental Changes ===
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During the last 5 days of gestation, there are changes in the placenta.
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* Collaginisation of the placentome.
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* Flattening of maternal crypt epithelium.
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* Leucocyte migration and increased activity.
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* Reduction of binucleate cells in the trophectoderm.
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* Weakening of the acellular protein layer between cotyledonary and caruncular epithelium.
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=== Contractions ===
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* Open endometrial crypts
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* Foetal villi have shrunk due to the escape of blood from the foetal side of the placenta when the umbillical cord ruptures.
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* Myometrial contractions aid exsanguination of the placenta.
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* Separation of foetal membranes.
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* Apex of the allantochorionic sac becomes inverted.
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* As the sac is 'rolled' down the uterine horns, foetal villi are drawn out of the crypts.
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* When a large portion becomes detached and inverted, it forms a mass in the maternal pelvis.
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** Stimulates reflex contractions of abdominal muscles.
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** Completes expulsion of the allantochorionic sac.
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* In polytocious species, dehisence and expulsion of foetal membranes are interspersed with births of the young.
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** Only expulsion of the last afterbirth stimulates abdominal contractions.
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* The final stage of allantochorionic expulsion lasts 1 hour (mare) - 6 hours (cow).
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* Domestic animals normally eat the afterbirth.
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** Not the mare or camelids.
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* Expulsion of the foetus requires that the chorionic villi become dislodged from the crypts of the maternal side of the [[Female_Reproductive_Tract_-Placenta_-_Anatomy_%26_Physiology|placenta]].
 
* Expulsion of the foetus requires that the chorionic villi become dislodged from the crypts of the maternal side of the [[Female_Reproductive_Tract_-Placenta_-_Anatomy_%26_Physiology|placenta]].
 
** Brought about by powerful vasoconstriction of arteries in the villi.
 
** Brought about by powerful vasoconstriction of arteries in the villi.
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