Kufa University
Veterinary Medicine College
Theriogenology
Obstetrics
Majid .A .Talal
The
parturition
Parturition:is an endocrine event, dependent upon the activation of the fetal
hypothalamus–pituitary–adrenal
(HPA)
axis,
as
was
demonstrated by the classical experiments of Liggins and
coworkers in the early 1970s (e.g. Liggins et al., 1973) in the
sheep. It is assumed that similar mechanisms operate in cattle.
For example, in the pregnant cow, plasma concentrations of
corticosteroids rise about 15-fold during the last 20 days of
gestation. This would appear to be important in the initiation of
parturition, since the infusion of synthetic adrenocorticotrophic
hormone (ACTH) into fetal calves also results in parturition
within seven days. This could help to explain why twin calves
tend to be born somewhat prematurely. The combined output of
both sets of adrenal glands would result in threshold levels of
corticoids being reached sooner
Physical changes associated with parturition
Parturition or birth is classically considered to occur
in three stages of labour:
1
• First stage – the preparatory stage, during which time the
pelvic ligaments slacken and the cervix dilates.
• Second stage – expulsion of the fetus through the pelvic
canal
• Third stage – expulsion of the fetal membranes and initial
involution of the uterus
First-stage labour
First-stage labour consists of the preparation of the mother and
fetus for the actual birth process. During this time regular
contractions of the myometrium begin at a rate usually of 12–24
contractions per hour (Gillette & Holm, 1963).
The cotyledonary attachments of the placenta begin to ‘loosen’
and the cervix shortens and dilates, partly due to the
contractions, but also due to the breakdown of the collagenous
tissue. There are often signs of discomfort, with the
cow tending to bellow and possibly kick at the abdomen; she
may also be restless and wander away from the rest of the group
if at pasture or loose-housed. Also, the back is often arched and
the tail raised. During this period the calf alters its disposition in
that its forelimbs become extended in preparation for birth
(compare calf disposition in Figs 11.11 and 6.2). The length of
first-stage labour is usually between 6 and 24 hours, tending
to be shorter in older, parous cows.
Second-stage labour
This is characterized by the onset of regular contractions of the
abdominal muscles compressing the abdominal contents. The
myometrial contraction frequency increases up to about 48 per
hour with 8–10 abdominal contractions occurring for each
myometrial contraction.The cow will usually become recumbent
2
during second-stage labour. The myometrial contractions force
the fetus backwards from the abdominal cavity into the pelvic
cavity, which in turn causes the abdominal contractions
Fig. 6.2 Disposition of the calf immediately prior to birth.
(Redrawn from Salisbury
et al., 1978.)
(straining). The pressure of the fetus against the cervix and
anterior vagina stimulates the release of oxytocin from the
posterior pituitary gland, which in turn stimulates further
contractions of the myometrium. This mechanism is a typical
neuroendocrine reflex arc and is known as Ferguson’s reflex.
The allantochorion is often ruptured quite early during secondstage labour with the escape of fluid through the vulva. As
contractions proceed the amnion or water bag appears at the
vulva and the calf’s front feet are soon apparent inside. The
amniotic sac may or may not rupture and, if it does, it provides
lubrication to
the calf’s passage through the birth canal. After the calf’s head
has been expelled, abdominal contractions may cease for a short
period before the rest of the body and lastly the hind limbs are
expelled. The umbilical cord usually
breaks spontaneously during expulsion of the fetus. Secondstage labour is usually completed in between 0.5 and 4 hours.
Third-stage labour
After expulsion of the fetus abdominal contractions cease;
however, myometrial contractions continue, resulting in the
separation and expulsion of the fetal membranes. This process
may take up to 6 hours, but if longer than 24 hours it is likely to
be due to a pathological cause.After the fetal membranes have
been expelled, myometrial contractions continue as well as the
release of oxytocin and PGF2a. These factors result in an initial
rapid rate of reduction in the size of the uterus. The pregnant
horn is usually halved in diameter by about day 5 postpartum
3
and halved in length by day 15. After this period the rate of
involution is reduced, but in normal cows can be considered to
be complete by day 30 postpartum
4