Page 513 - Anatomy and Physiology of Farm Animals, 8th Edition

498 / Anatomy and Physiology of Farm Animals

useful for pregnancy diagnosis and esti- abdominal contraction forces the fetus
through the birth canal. This period is
mation of the stage of pregnancy. In the
VetBooks.ir cow, the presence of a corpus luteum in an often very rapid in domestic species, with
most foals being born within 15 to 30
ovary and a slight enlargement of one uter-
ine horn as compared to the other suggest minutes.
an early pregnancy. At about 3 months of The third stage of parturition consists
pregnancy in the cow, fetal membranes of delivery of the placenta, which normally
and caruncles become palpable, and the follows the fetus almost immediately.
uterine artery on the side with the fetus is
slightly larger than the vessel on the other Late Gestation
side. Pregnancy diagnosis by palpation per
rectum is considered to be more difficult in To facilitate delivery of the fetus, muscles
the mare than in the cow, but an early diag- and ligaments of the birth canal relax
nostic feature is a bulge in the uterus due to shortly before parturition. The vulva
the development of the amniotic sac. swells, and a mucus discharge may be pre-
Ultrasonography is used to diagnose
pregnancy in a variety of domestic species, sent. Muscles on both sides of the tail head
may relax and appear lowered or depressed.
including cattle, horses, sheep, goats, llamas, Mammary glands enlarge and may secrete
and swine. The earliest time for verifica- a milky material for a few days prior to
tion of pregnancy is dictated in part by the parturition. As the time of parturition
size of the embryo, which in turn varies becomes imminent, animals may become
among species. In general, the times vary restless, seek seclusion, and increase the
between about 2 weeks for mares to about frequency of attempts to urinate. The bitch
5 weeks for ewes. In large animals, the and sow often try to build a nest.
ultrasound probe can be inserted via the An important endocrine change during
rectum so that it is closer to the uterus. As late gestation in most species is in the ratio
gestation continues, the developing fetus of estrogen to progesterone. Progesterone
and gravid uterus will descend within the is high relative to estrogen during most of
abdomen and may require transabdominal gestation, but this ratio changes during late
ultrasonography for evaluation during late gestation, with estrogen increasing relative
gestation.
to progesterone. Estrogen promotes the
development of contractile proteins in
Parturition the smooth muscle cells of the uterus and
gap junctions between these cells. These
uterine changes increase the force that
Parturition (labor), the act of giving birth the uterus can generate for delivery. The
to young, marks the termination of preg- timing of the changes in estrogen and pro-
nancy. Parturition may be divided into three gesterone relative to parturition varies
stages. The first stage consists of uterine among species.
contractions that gradually force the fetus
and fetal membrane to the cervix. The dura-
tion of this stage is a matter of hours in most Initiation of Parturition
species (e.g., 2 to 6 in the cow and ewe, 1 to 4
in the mare, and 2 to 12 in the sow). In domestic animals, an endocrine signal
In the second stage, actual delivery of from the fetus appears to initiate parturi-
the fetus occurs. Passage of parts of the tion. Plasma levels of glucocorticoids (e.g.,
fetus through the cervix into the vagina cortisol) increase in most domestic spe-
along with rupture of one or both water cies, except for the mare, shortly prior to
bags reflexively initiates actual straining or parturition. The fetal adrenal cortex is the
contraction of the abdominal muscles. The source of these glucocorticoids, and the
combination of uterine contraction and adrenal cortical secretion is in response to