Normal Parturition - Anatomy & Physiology

The Process by which the conceptus (foetus, placenta and placental membranes) are expelled from the Uterus requires:
- Cervical Softening
- Coordinated Myometrial Contraction
- Voluntary contraction of abdominal muscles

Placental Expulsion

In most species, expulsion of foetal membranes quickly follows expulsion of the foetus.

After the birth of the young, regular abdominal contractions largely cease.
Myometrial contractions persist.
- Decreased amplitude, but become more frequent and less regular.
- Important for dehiscence and expulsion of foetal membranes.
Waves of contractions from uterus to the cervix persist.
- Act in a peristaltic fashion in the cow and sow in the reverse direction.

During the last 5 days of gestation, there are changes in the placenta.

Collaginisation of the placentome.
Flattening of maternal crypt epithelium.
Leucocyte migration and increased activity.
Reduction of binucleate cells in the trophectoderm.
Weakening of the acellular protein layer between cotyledonary and caruncular epithelium.

Open endometrial crypts
Foetal villi have shrunk due to the escape of blood from the foetal side of the placenta when the umbillical cord ruptures.
Myometrial contractions aid exsanguination of the placenta.
Separation of foetal membranes.
Apex of the allantochorionic sac becomes inverted.
As the sac is 'rolled' down the uterine horns, foetal villi are drawn out of the crypts.
When a large portion becomes detached and inverted, it forms a mass in the maternal pelvis.
- Stimulates reflex contractions of abdominal muscles.
- Completes expulsion of the allantochorionic sac.
In polytocious species, dehisence and expulsion of foetal membranes are interspersed with births of the young.
- Only expulsion of the last afterbirth stimulates abdominal contractions.
The final stage of allantochorionic expulsion lasts 1 hour (mare) - 6 hours (cow).
Domestic animals normally eat the afterbirth.
- Not the mare or camelids.

Expulsion of the foetus requires that the chorionic villi become dislodged from the crypts of the maternal side of the placenta.
- Brought about by powerful vasoconstriction of arteries in the villi.
  - Vasoconstriction reduces pressure and allows villi to be released from crypts.

Species	Mare	Cow	Ewe	Sow	Bitch
Stage 1: Contractions and Cervical Dilation	1-4 hours	2-6 hours	2-6 hours	2-12 hours	6-12 hours
Stage 2: Foetal Expulsion	12-30 minutes	30-60 minutes	30-120 minutes	150-180 minutes	6 hours (up to 24 hours with large litters)
Stage 3: Placental Expulsion	1 hour	6-12 hours	5-8 hours	1-4 hours	Placenta Exits with Foetus