Stages of Parturition (Labour)

Stages of parturition or labour do not start or end abruptly and there is no clear demarcation of the stages. But gradually passed from one stage to other stage.

The three stages of labor in animals include:

1. 1^st stage of labour (Cervical dilatation)
2. 2^nd stage of labour (Foetal expulsion)
3. 3^rd stage of labour (Expulsion of foetal membranes)

1. Stage of Cervical Dilatation

Stage of cervical dilatation marks the onset of parturition, but the changes are not visible externally. These changes are essential to prepare the birth canal and foetus for expulsion of foetus and foetal membranes.

The changes comprises of:

• Myometrial contraction
• Cervical dilatation
• Foetal disposition

Myometrial Contraction

Myometrial contraction is irregular, in-coordinated contractions occurs during last month of gestation which will be converted to regular, rhythmic, coordinated and peristaltic contractions at the time of 1st stage of labour.

Uterine contractions start at the apex of the cornua and extend caudally. It is initiated by the contractions of circular muscle fibres and is propagated by the longitudinal muscle fibres. About 90% of the parturition process is carried out by the uterine contraction and 10% by abdominal contraction.

Contraction amplitude, frequency and duration increases at the time of parturition. Uterine contraction occurs at every 15–30 minutes and it persists for 15-30 seconds. Just 1–2 hours, prior to labour, the frequency increases as 12 contractions/hour and at the time of expulsion of foetus 24 contractions/hour.

In uniparous animals, the cranial portion of the uterus contracts, whereas caudal portion of the uterus and cranial portion of cervix dilates. The average amplitude during this stage is 80 cm of H₂O.

Cervical Dilatation

Cervical dilatation is brought about by active and passive process. Active process means contraction of the uterine musculature and contraction of longitudinal muscle fibres causing cervical dilatation.

Passive process means the pressure exerted by foetus and foetal membrane on the cervix cause cervical dilatation. The external os of cervix dilates first and followed by internal os. Cervix dilates in the form a cone and base of the cone located cranially and will allow one hand to pass.

True dilation of cervix occurs 2-4 hours after the dilatation of the external os. At this time, the dilatation of the external os is about 3”–6”. Complete cervical dilatation occurs within 2-6 hours. At this time the cervical dilatation is about 6-8”. The cervix and vagina becomes a continuous canal at the time of parturition.

The dilatation of internal os is faster than the external os. Full dilation of the cervix occurs when the foetus and foetal membranes occupies the half of the vagina.

Foetal Disposition

In case of foal and puppies, there is a progressive rotation from the ventral (dorso pubic) to the dorsal (dorsosacral) position while the fore limbs, head and neck becomes extended. In cattle, buffalo, sheep and goat, the flexed knees of the foetus first occupied the dilating cervix and 30 minutes after the digits are in the cervix then it extends its carpel joint.

Thereby try to “stand up in utero” and it extends the extremities towards the birth canal in a forward direction called “Righting reflex” which is a unique attitude in bovine species and never repeated after birth. The changes in the fetal movement posture are brought about by myometrial contraction.

In polytocous species, the uterine contraction starts just cranial to most caudal foetus. There is no regularity in the expulsion of foetus.

Expelling of subsequent fetus may from the same horn or most caudal foetus from the opposite horn. After expulsion of each foetus, the longitudinal muscle fibres of the uterine segment which carried the expelled fetus will contracted and reduced the length. So the last foetus in the uterine horn need not travel the entire length of the uterus.

During this period, the panting may be noticed in sows and vomiting in bitches and reduced the body temperature about 1–2°F. In mares, rolling of the dam will facilitate the turning of foetus from its dorso-pubic to dorso-sacral position.

Towards the end of 1st stage, the allanto-chorionic sac ruptures, the amnion and foetal limbs enter into the birth canal, which produces a neural stimuli leads to release of the oxytocin and abdominal contraction starts.

Duration of 1st Stage

• Cattle: 2–6 hours
• Sheep & Goat: 2–6 hours
• Dog, Cat & Swine: 1–12 hours
• Mare: 1–4 hours

2. Stage of Foetal Expulsion

Stage of foetal expulsion is characterized by rupture of allanto-chorionic sac, abdominal straining, and entrance of the fetus into the birth canal and expulsion of foetus.

When the fetal extremities touch the cervix and vagina, the furgusions reflex stimulates the release of oxytocin from the posterior pituitary and stimulates the uterine contraction which pushes the fetus into the pelvic cavity and produces the abdominal straining.

Oxytocin is released continuously at the time of parturition. The level of oxytocin varies from species to species. Goat 77-381 micro units/ml of plasma, sheep 3000 micro units/ml of plasma and cattle 40-1000 micro units/ml of plasma.

After the rupture of allanto chorionic sac, severe abdominal contraction started and the amnion appears at vulva, (2nd or true water bag) as a distended translucent membrane. Within the amnion, the feet of foetus can be seen. Rupture of amnion releases syrupy, gummy fluid and feet of foetus will be appearing at vulva. One foot proceeds the other foot about few inches behind.

Uterine contraction during 2nd stage of labour is 80–320 cm of H₂O (average 180 cm of H₂O) and the intrauterine pressure is 70 mm of Hg and abdominal contraction is 170 mm of Hg, which is equal to one man traction of the foetus. Here, the presence of healthy foetus and uterus and intact abdominal wall are necessary for normal parturition.

In case of uniparous species, the fetus passes the birth canal in the form of an ‘’arc’’. ‘’arc’’ like fashion causes stretching of the dorsal and pelvic muscle and relaxation of the linea alba and abdominal muscles thus reduces the pelvic diameter of foetus. Also the fetal hip is elevated and pushes the fetal pelvis high in the maternal pelvis, where the bisiliac diameter is greater. This helps to prevent a hip lock condition.

After the expulsion of foetus, the umbilical arteries are retracted into abdominal tissue of the foetus, umbilical vein collapses, urachus retracted into the abdominal tissues and umbilical cord become necrotic and drops away in 7-21 days.

Mare

Parturition process is very quick in mares, 2-5 abdominal contractions followed by 2-3 mts rest. During foaling, one limb follows the other limb by 6 inches behind. Following the expulsion of foetus, the dam is exhausted and lye on the ground for 15-30 mts. Mare prefers to deliver the young one in lateral recumbency with extended limb. Since, the umbilical cord is longer, it remains attached with uterus, gets ruptured 2” away from the abdomen of the foal. Pulses can be felt in umbilical cord for 1-8 mts and immediate rupture results in loss of 400–1500 ml of blood from the foetus.

Sheep & Goat

Similar to cattle.

Bitch, Cat and Sow

When the amnion reaches the vulva, the bitch licks the amnion and cause rupture. When foetus touches vagina, strongest and greatest contraction occurs. Within the minutes of delivery of a pup placental membranes are expelled and then next pup is expelled. But there is no regularity in expulsion of foetuses in bitches.

The interval between foetal expulsion is sow is 10-40 minutes. Sow urinates copiously after expulsion of all foetuses.

At the time of expulsion of foetal membrane greenish black discharge is noticed in bitches. This is a normal colour of lochia and is due to break down of haemoglobin in the blood to uteroverdin which is a bile like pigment.

Bitch may eat 1 or 2 placenta and the habit of eating placenta is called “placentophagy”, but mare will never eat the foetal membranes. The sow may eat or 2 fetuses and the habit is called as “foetophagy”. Presence of dead foetus may delay the parturition process.

Duration of 2nd Stage

• Primipara cow: 0.5–3 or 4 hours
• Pluripara cow: 0.5–1 hours
• Sheep and Goat: 0.5–2 hours
• Bitch and Cat: 6 hours (extended upto 12 hours)
• Sow: 1–5 hours (extended upto 8 hours)

3. Stage of Expulsion of Foetal Membranes

Stage of expulsion of foetal membranes, comprises of the expulsion of foetal membranes and uterine involution.

Expulsion of foetal membranes takes place within few hours after delivery of the foetus and uterine involution takes more than a month. In case of polytocous species 2^nd and 3^rd stage are merged.

After the expulsion of the fetus, dam takes rest and uterus continues to contract strongly upto 48 hours but thereafter it becomes less vigorous but more frequent. Uterine contraction produces following changes:

• It reduces blood flow to the endometrium
• It reduces the size of the uterus
• It pushes the foetal membrane towards the birth canal
• It causes separation of villi from maternal crypts

The exact mechanism of placental separation is not known. It is a complex process contributed by mechanical and hormonal processes. Mechanism of placental separation is due to the reduced uterine blood flow, dilatation of maternal crypts and shrinkage of foetal villi. This play an important role in the placental separation. Each uterine contraction will be superimposed upon the abdominal contraction.

Placental separation starts from the apex of the cornua (horn), hence the shiny allantoic surface will be expelled out first. Following the expulsion of part of foetal membranes, it will exert pressure in the birth canal and causes abdominal straining and complete expulsion of foetal membranes occurs. The expulsion of white cartilaginous tissue indicates complete expulsion of foetal membranes. Following expulsion of foetus membranes, absence of whirring or fremitus in middle uterine artery is noticed.

Duration of 3rd Stage

• Cattle: 0.5–8 hours
• Ewe: 0.5–8 hours (6 hours)
• Mare: 0.5–3 hours

In Bitches and Cats

Both the foetuses and the foetal membrane will be expelled out or one foetus along with foetal membrane of a previous foetus will be expelled out or foetus along with its own foetal membrane will be expelled out.

In Sows

Allanto-chorionic surfaces of foetal membranes are attached, hence foetal membrane are expelled at 2-3 intervals. After the expulsion of last foetus, within few hours, the entire foetal membrane will be expelled out.