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.
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