Page 514 - Anatomy and Physiology of Farm Animals, 8th Edition

Pregnancy and Parturition / 499

adrenocorticotropic hormone (ACTH) Fetal Presentations and Delivery
from the fetal adenohypophysis. An
VetBooks.ir increase in sensitivity of the adrenal cortex The calf is normally presented front feet

to ACTH is part of the reason for the
increased secretion of glucocorticoids dur- first with the head extended and the nose
between the front feet (Fig. 28‐7). The
ing this period. dorsum of the calf is in contact with the
The rising fetal glucocorticoids affect sacrum of the dam. This position, called
the placenta and the maternal uterus. In cranial (anterior) presentation, takes
some species, the glucocorticoids increase advantage of the natural curvature of the
estrogen production by the placenta so birth canal of the dam and the curvature of
that plasma estrogens increase relative to the fetus. A caudal (posterior) presentation
progesterone. This brings about the with the hind feet first, hocks up, occurs
changes in the uterine smooth muscle frequently enough in cattle to be consid-
described earlier. Glucocorticoids and ered normal.
estrogens act synergistically to promote Uterine contractions force the fetal
uterine synthesis and secretion of PGF α. placenta (water bags) against the cervix of
2
Increases in PGF have multiple the uterus, and ultimately these water bags
2α
effects that can contribute to the onset of rupture. At about the same time, abdomi-
parturition, and some view these as the nal muscles begin to contract forcefully to
final link in the chain of events initiating expel the fetus through the birth canal.
parturition. In species whose primary The contraction of abdominal muscles,
corpus luteum remains and is a signifi- called straining, is a reflex response to
cant source of progesterone (e.g., cow stimuli from the presence of parts of the
and sow), the rise in PGF can bring fetus within the vagina and vulva of the
2α
about leutolysis and remove this source dam. Straining can also be readily evoked
of progesterone. Recall that progesterone by the insertion of a hand and arm into the
suppresses the activity of uterine smooth vulva and vagina, for example when
muscle and maintains cervical tone, so a attempting to deliver a calf.
reduction in progesterone is appropriate The legs of a foal are longer than those
to facilitate delivery. PGF also directly of a calf, and a foal is carried to a larger
2α
stimulates the contraction of uterine
smooth muscle to move the fetus into the
birth canal and directly promotes dilation
of the cervix.

Oxytocin

The entrance of a fetus into the birth canal
brings about a reflex increase in oxytocin
secretion from the neurohypophysis.
Oxytocin acts directly on uterine smooth
muscle to enhance uterine contractions
and promote delivery. Extracts of the neu-
rohypophysis are used clinically to stimu-
late contractions of the fatigued uterus
during prolonged labor.
In the mare, plasma levels of oxytocin
gradually rise during the latter stages of
gestation and then increase greatly as Figure 28-7. Cranial or anterior presentation
parturition begins. of a calf.