CHAPTER 11 Pregnancy Evaluation in the Mare
Effective management of broodmares requires that their pregnancy status be known so that proper managerial steps can be taken to ensure optimal care. At 14 to 18 days after ovulation, a mare that has been bred should be carefully examined for pregnancy; if the mare is not pregnant, steps can be taken to have her bred prior to the next ovulation. Early detection of twins is imperative for a successful reduction to a single pregnancy by manual crushing of one conceptus. If performed after 30 days of gestation, the procedure is less likely to result in a single, viable fetus. Ideally, mares that have been pronounced in foal should be teased three to four times per week and re-examined at approximately 30, 42, 60, and 120 days and again in the fall to monitor their reproductive status. If pregnancy loss has occurred, they can be rebred expeditiously if no obvious cause is detected that requires treatment. Broodmares that are not pregnant at the end of the breeding season should undergo a breeding soundness examination. Therapy, if indicated, can then be instituted and the mare’s reproductive status be re-evaluated prior to the onset of the next breeding season.
Knowledge that a mare is pregnant allows one to make the decision to vaccinate for protection against equine herpesvirus I (rhinopneumonitis) periodically throughout gestation (3, 5, 7, and 9 months) and to administer other vaccines as appropriate for the geographic location 1 month prior to the mare’s anticipated due date. Use of anthelmintics should be avoided the first 60 days of gestation during organogenesis, as well as during the last 30 days of gestation, when any abortion/stillbirth that might occur may be attributed to their use. Otherwise, appropriate anthelmintics approved for use during pregnancy (consult the product label and package insert carefully) should be administered periodically throughout gestation. The frequency of anthelmintic administration will be determined by the environment and animal density, but may be as often as every 6 to 8 weeks or even on a daily basis. During the last trimester, pregnant mares may need to be separated from open mares so that additional feed can be provided as necessary to maintain optimal development of the fetus. The pregnant mare herd should be isolated from any transient population to prevent exposure to infectious diseases. Mares greater than 300 days pregnant should be closely observed for signs of mammary gland development and impending parturition.
Several techniques are available to determine the presence of an embryo or fetus and the status of pregnancy. Some techniques are used to directly evaluate the conceptus or fetus; other techniques are used to indirectly assess the conceptus by evaluating changes in the mare’s behavior, hormones, or genital tract that are due to pregnancy. The procedure used will be based on factors such as stage of gestation, temperament, size, reproductive or medical history of the mare, examination facilities, equipment available, intensity of management, financial constraints of the owner, value of the mare, and experience of the veterinarian.
Although the absence of estrus may suggest the pregnancy status of some species, it is not a reliable indicator in the mare. Ten percent of pregnant mares may show estrus during pregnancy. Estrus during pregnancy typically occurs around 18 to 21 days after ovulation. Mares that experience pregnancy loss between 40 to 120 days of gestation typically do not come back into estrus until after the endometrial cups have involuted. Nonpregnant mares would likely fail to display estrus during the winter anestrous season.
PALPATION PER RECTUM
Characteristic changes occur in the mare’s genital tract during pregnancy. Some changes are due to the response of the genital organs to hormone stimulation. Changes in dimensions of the uterus and the position of the genital tract within the pelvic canal are caused by an increase in size of the growing conceptus and increase in fetal fluid volume. The large size and tolerant nature of most broodmares permit evaluation of changes in the internal genitalia by palpation per rectum.1 Careful, systematic palpation of the internal genital tract per rectum requires generous lubrication, gentle removal of feces, and adequate restraint of the mare. To prevent damage to the rectal mucosa, the tail should be wrapped or care taken to avoid inadvertently dragging tail hairs into the rectum during the examination. A water-soluble methylcellulose lubricant is recommended because it is not irritating to the rectal mucosa. The lubricant should be rinsed or wiped from the perineum after the examination to prevent chapping of the perineum. The rectum must be emptied of feces so that the genital organs can be felt. Mares can be palpated per rectum while being held in hand, standing in a stall doorway or in stocks. A nose twitch may be required to more safely restrain restless or difficult mares. As with any exogenous medications, administration of tranquilizers should be avoided during pregnancy. Nonetheless, if the results of the examination are critical for making management decisions and there is risk of injury to the mare or examiner, chemical restraint of the mare may be necessary. Adequate relaxation may be achieved using acepromazine (0.05 mg/kg IV) alone. Xylazine (0.3–0.5 mg/kg IV) combined with acepromazine (0.05 mg/kg IV) has been used clinically to help nervous mares tolerate palpation per rectum. Although xylazine has little effect on fetal heart rate, it causes contraction of the uterus, increases uterine tone, and may induce premature parturition. Therefore, xylazine should be used with caution during the last trimester of pregnancy.
As early as 16 to 18 days after ovulation, the cervix of the pregnant mare becomes tightly closed, firm, slender, and elongated. The portio vaginalis assumes a pointed shape. In multiparous mares, the cervix may not assume this character until 21 days after ovulation. These changes are not only due to elevated blood concentrations of progesterone but also due to “embryonic” factors not yet identified but thought to be associated with pregnancy. The cervix of the normal pregnant mare may remain closed until the mare goes into first stage labor, or it may soften and relax during the month prior to parturition.
Careful evaluation of the uterine horns between 12 and 25 days of gestation will reveal a marked and gradual increase in uterine wall thickness. The endometrial folds are no longer palpable as folds of tissue rippling through one’s fingertips. Instead the uterus becomes tubular, smooth, and firm with tone. This tone is caused by progesterone and embryonic factors. Distinct bends at the base of the uterine horns may be present at 20 to 22 days of gestation. The conceptus becomes positioned at the base of one of the uterine horns near the junction of the uterine body. Both the nongravid and gravid uterine horns on each side of the conceptus maintain a firm, tubular character as the conceptus increases in size. The uterine wall thins over the developing conceptus, which maintains a buoyant, vibrant quality throughout gestation. The uterine horns and body gradually lose the tubular tone after 48 to 55 days.
The conceptus develops in a recognizable pattern of size and shape, allowing an estimation of age based on its palpable characteristics. In maiden and barren mares, a careful, experienced clinician may be able to feel the embryonic vesicle ventrally at the base of one uterine horn at 17 to 25 days, producing a bulge 3.5 cm in diameter. At 30 days of gestation, both uterine horns will be small with prominent tone and the conceptus can be felt as a ventral bulge 4 cm in diameter and positioned at the base of the gravid uterine horn. The uterine wall becomes thinner over the expanding conceptus. At 42 to 45 days, the conceptus occupies approximately half of the gravid uterine horn and is 5 to 7 cm in diameter. The enlargement of the conceptus begins to involve the uterine body by 48 to 50 days and is 6 to 8 cm in diameter and 8 to 10 cm long. At 60 days of gestation, nearly the entire gravid horn and half of the uterine body are filled with conceptus, but the nongravid horn remains small. The 60-day conceptus is 8 to 10 cm in diameter and 12 to 15 cm in length. After 85 days, the turgidity of the conceptus decreases such that the fetus becomes palpable. At 90 days of gestation, the conceptus fills the entire uterus and the cranial portion of the uterus may extend over the brim of the pelvis into the abdominal cavity. After 100 to 120 days, the gravid uterus will be positioned cranial to the pelvic brim in the abdominal cavity. The ovaries will be positioned toward the midline, cranial and ventral to their normal nonpregnant position because of the ventral traction the enlarging uterus exerts on the broad ligament.
Expansion of the uterus is not a gradual filling process but rather alternately proceeds and recedes, apparently in response to transient segmental constrictions of the uterus. These constrictions seem to cause a reallocation of allantoic fluid. Reclosure of the uterine horns first occurs after day 106. Although the placental membranes interdigitate with the endometrium of the uterine horns, little fetal fluid remains in the uterine horns. At 150 days, palpation of the fetus is still possible. Between 5 to 7 months, the uterus is positioned ventrally in the abdomen and it is difficult to thoroughly evaluate the conceptus by palpation per rectum. From 6 to 7 months the horns are approximately perpendicular to the dorsal cranial aspect of the uterine body. As the conceptus continues to grow, it expands in a dorsal direction so that by 11 months the mesometrial surface of the uterine horns move closer to the dorsal surface of the uterine body and the fetus is within reach of the examiner when palpating per rectum.
Normally the fetus is active after 40 days and mobile after 70 days of gestation.2 Fetal activity or movement of the head, mouth, and limbs and fetal mobility occur throughout the entire fetal stage of gestation. If one fails to immediately detect fetal movement during palpation per rectum in a later stage of pregnancy, it is advisable to be patient in assessing the presence of fetal activity. Several minutes may be necessary for a movement to be noted.
During early gestation (2 to 5 months) the presentation of the fetus changes owing to the fetal mobility. As fetal mobility decreases after 4 months of gestation the fetus is more likely to be in a cranial presentation. During the seventh to eighth month the hind limbs of the fetus enter the uterine horn containing the umbilical cord, and although the limbs remain active throughout gestation, they do not retract from this horn. Distinct parts of the fetus may not be discernible, and thus it is difficult to accurately assess fetal presentation solely by palpation per rectum. Fortuitous visualization of the orbit during an ultrasound examination per rectum or location of the heart and stomach transabdominally with ultrasonography will permit a more accurate assessment.
The ovaries are active during early gestation. The primary corpus luteum forms from the follicle that ovulated the oocyte of the pregnancy and persists for 160 to 180 days. Numerous medium and large follicles are present on the ovaries between 40 and 60 days. Secondary corpora lutea form from ovulation and luteinization of these follicles. After 180 to 200 days, there is little ovarian activity and evaluation of the ovaries by palpation per rectum after 120 days is difficult because the ovaries are pulled ventrally under the broad ligament because of the weight of the enlarging gravid uterus.
VAGINAL SPECULUM EXAMINATION
Reproductive status can be ascertained without invading the internal genital tract by palpation and ultrasonography per rectum as well as by transabdominal ultrasonography. Nonetheless, reproductive status can be ascertained by evaluation of the cervix of the mare by direct visualization through an illuminated speculum. Speculum examination requires the mare to be restrained as for palpation per rectum. The tail should be wrapped to prevent hairs from entering the vaginal canal. The perineum should be washed with povidone-iodine scrub or mild soap, thoroughly rinsed, and dried. A sterile speculum, lubricated with sterile, water-soluble lubricant, is gently passed through the vulvar lips, vestibule, vestibular ring, and caudal vagina. An external source of light allows one to visualize the cranial vagina and the portio vaginalis of the cervix. It is not possible to prevent air from passing through the speculum, which may allow the introduction of microorganisms or induce straining by the mare. Iatrogenic pneumovagina is particularly a problem in thin mares, older or swaybacked mares, and mares with an incompetent vestibulovaginal ring. Manipulation (stimulation) of the cervix, particularly in late gestation, should be avoided because it may result in prostaglandin release and premature parturition.
The appearance of the cervix is directly related to the hormone milieu. During pregnancy, the cervix will be tightly closed by the influence of progestogens and unidentified substances produced by the conceptus. The portio vaginalis is yellow-gray, dull, dry, sticky, and in a central position in the cranial vagina. During the last month of pregnancy, the normal cervix may be soft or relaxed, so great care must be taken not to initiate an iatrogenic ascending infection.