Normal Parturition - Anatomy & Physiology

Stages of Parturition

• Stage 1: this is the preparatory stage, starting at the onset of regular uterine contractions followed by cervical dilatation and the foetus assuming the correct disposition for passage through the birth canal.
• Stage 2: the expulsive stage, characterised by the onset of abdominal contractions which together with uterine contractions lead to foetal expulsion
• Stage 3: separation and expulsion of the foetal membranes. In polycotous species second and third stages are often inter-mixed.

The following table summarises the normal time taken to progress through the stages of parturition in different species.

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 minutes - 4 hours	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

The step in the reproductive process that immediately precedes lactation, uterine involution and return to cyclicity. It is initiated by the foetus and involves a complex cascade of endocrine events. Parturition is the process by which the conceptus (foetus, placenta and placental membranes) is expelled from the uterus; this requires cervical softening,coordinated myometrial contractions and contraction of abdominal muscles to occur

Placental Changes

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.

Contractions

• 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.

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.