Normal Parturition - Anatomy & Physiology
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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.
|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
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.
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.
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