Gestation and Parturition

Gestation lasts 280 days in cattle, 147 days in sheep, and 154 days in goats. During this time every part of the reproductive system shows some changes, but obviously the most striking are in the uterus, which increases its weight 15-fold (100-fold when its content is included).

The ovary is distinguished by the presence of the corpus luteum of pregnancy, which persists beyond the life span of the infertile cycle. Its survival is not always accompanied by total suppression of follicular activity; a few cows come into heat and ovulate in early pregnancy. The corpus luteum is not necessary for the support of pregnancy during the past 3 months and usually begins to regress about a month before term (Figs. 29.20 and 29.21).

FIG. 29.21 Transrectal color Doppler ultrasonography of the ovary (outlined by arrowheads) showing corpus luteum in a cow on Day 65 of gestation. Notice the marked corpus luteum blood vessels (color areas) originate from main vessels at end one spot (1), spread along the periphery of the corpus luteum (2) giving rise to radial vessels (3) that supply the glandular tissue (4). Black areas (5) represent small antral follicles in the cortical area. Scale bar = 10 mm.

The progestational changes that are part of every cycle persist and intensify in the presence of an embryo. Although the blastocyst is initially confined to one horn, the membranes soon spread to the other; however, the embryo, and later the fetus, is almost invariably restricted unilaterally, and a developing asymmetry is one of the first detectable signs of pregnancy. The amniotic sac becomes palpable about the 30th day, the fetus itself about the 70th. The caruncles of the gravid horn gradually increase from low, smooth-surfaced bumps to become large, pedunculated swellings with surfaces pitted for the reception of the chorionic villi; by term the largest may attain the size of a clenched fist (Figs.

The enlargement of the uterus does not affect all parts equally. The lesser curvature, being tethered by the broad ligament, most resists expansion, which causes the horn to alter shape: the greater curvature and adjacent parts grow away from the attachment. Hypertrophy of the tissues of the broad ligament restrains the uterus from sinking into the abdomen for a time, but by the third month this resistance is overcome and the uterus begins to slip forward over the abdominal floor. The supply of blood to the gravid uterus by all vessels but especially the uterine artery, which expands from a few millimeters to a centimeter or more, is necessarily greatly increased. The uterine artery loses its flexuous character and now passes forward into the abdomen, where it is easily identified based on characteristic vibrations (fremitus) on palpation against the ilium.

FIG. 29.22 Transrectal ultrasonographic sections of the the placentome (left), fetal head (middle), and caudal part of female fetus (right) at 65 days of gestation. Notice the rich blood flow (color areas) to the placentome (1) in the figure on the left. Amniotic membrane (2) separates the fluid-filled amniotic (3) and allantoic (4) cavities. If fetus is not easily accessible, fetal membranes and placentomes can be used to differentiate pregnancy from other fluid collections in the uterus. Notice the size of the placentome in comparison to the fetal head (5) and neck (6). Fetal cranium is cartilaginous, showing clear demarcation (arrow) between the two halves. Eye sockets (7) are prominent. In the female fetus, genital tubercle (8) is located under the tail (9) and is used clinically to performed gender determination at 2 months of pregnancy (in male fetus, genital tubercle is present close to the attachment of umbilical cord).

FIG. 29.23 (A) A gravid uterus, partly opened. (B) A bovine fetus within its membranes. The villi are mainly restricted to the cotyledons. 1, Caruncle; 2, cotyledon.

FIG. 29.24 Paramedian section of the caudal abdomen and pelvis of a pregnant cow. The section is not quite vertical because it cuts through the vertebral canal and an obturator foramen. Note the large placentomes. 1, Sacrum; 2, rectum; 3, anal canal; 4, uterus; 5, cervix; 6, vagina; 7, vestibule; 8, bladder; 9, urethra; 10, suburethral diverticulum; 11, vulva.

The topography is not the same in every pregnancy. The enlarging uterus usually enters the supraomental recess but sometimes may slip forward against the right or the left flank. As it expands, it sinks within the abdomen and for a time passes out of reach of a hand within the colon; this inability to reach the uterus at about the fifth month is as diagnostic of pregnancy as its palpable enlargement at earlier and later times. The descent into the abdomen stretches the vagina and carries the cervix over the pubic brim. Toward term, the uterus occupies most of the ventral and right sections (in the common arrangement) of the abdomen, which raises the rumen dorsally and crushes the intestines upward (Fig. 29.24). It makes contact with the liver and diaphragm, on which it exerts increasing pressure. During the first months the calf enjoys freedom to move and adjust position within the amniotic fluid, but as pregnancy continues, it is forced to adapt to the form and dimensions of the uterine horn.

The cervical canal is closed by a mucous plug, developing from the first month and later projecting through the external cervical ostium.

Changes that signal the approach of parturition include softening of the sacrosciatic ligament, with insinking beside the tail head (Fig. 29.25A and B); a similar loosening of other pelvic ligaments allows some relaxation of the sacroiliac joints. The connective tissues of the cervix and caudal reproductive tract and vulvar and perineal skin share in these changes that, though spread over several weeks, are much intensified in the last few days. When parturition actually impends, edema of the soft parts may cause the vulva to gape.

FIG. 29.25 Indications of impending parturition. (A) Relaxation of the sacrosciatic ligament (arrow). (B) Swelling of the vulva (arrow).

The bony pelvis of cows is not particularly favorable to easy parturition. Its dimensions are relatively small, and the axis of the birth canal is broken where it passes over the pubic brim and again where the floor changes direction to rise toward the exit. Some increase in the vertical diameter is possible if the pelvis can rotate about the relaxed sacroiliac joints, but this relief is clearly denied to the standing cow. The principal soft tissue impediments to easy birth are the cervix, the caudal end of the vagina, and the vulva. Normally these parts also loosen under hormonal influence.

The umbilical cord ruptures when the cow gives birth and, being relatively short, often before delivery is complete. Its constituents part at different levels.

The fetal membranes (the "cleansings" in lay speech) normally separate from the endometrium and are expelled shortly after delivery; it is a process hastened by suckling, which stimulates the release of oxytocin.

Retention of fetal membranes (placenta) in utero may require human intervention to accomplish their removal. The retention of fetal membranes may lead to inflammation of the uterus (metritis) or mammary glands (mastitis) and consequent economic losses. The major reason for the retention of the fetal membranes is the lack of breakdown of the complex that attaches the fetal cotyledons to the maternal caruncles.

After parturition, the tract tends to return to its former state, but first pregnancies leave a permanent legacy in the form of thickening and loss of symmetry (Fig. 29.26). The uterus contracts as soon as it empties, undergoing a very rapid atrophy in which a third of its weight is lost within a couple of days; the second third is lost before the week is out. The decline is slower thereafter, but should a cow remain "empty," a period of superinvolution (lactation atrophy) may follow in which the size of the uterus drops below the resting norm. Involution of the vagina, vestibule, and vulva is slower.