CHAPTER 179 Pregnancy Diagnosis
Early, accurate diagnosis of pregnancy in the mare plays a critical role in determining her reproductive efficiency during a given breeding season. Once diagnosed pregnant, the mare can be monitored for continued appropriate embryonic growth and development. If the mare is not pregnant, earlier recognition enables reevaluation and less loss of time. In addition to broodmare management, accurate early diagnosis of pregnancy has several applications in equine practice, including evaluation of semen-fertilizing ability, decision making concerning flushing of embryo donors, easier and accurate evaluation of early embryonic loss, and evaluation of factors that affect embryo survival, particularly in assisted reproductive technologies.
For pregnancy to be established, a viable spermatozoa fertilizes a mature oocyte at the ampulla-isthmus junction of the oviduct. The embryonic vesicle is transported through the oviduct, where it enters the uterus via the uterotubal junction by day 5 to 6 after ovulation. Maternal recognition of pregnancy occurs between 12 and 16 days, when some aspect of conceptus mobility possibly prevents the release of or inhibits the function of prostaglandin F2α that would cause luteolysis, which results in retention of corpus luteum and maintenance of pregnancy. Pregnancy can be diagnosed by a variety of different methods, depending on the stage of gestation.
When a mare does not return to estrus 17 to 21 days after ovulation, the supposition may be that she is pregnant. Although this may be true, other factors can cause a longer than usual diestral period. These can include normal variability in an estrous cycle; a mare not exhibiting estrus behavior because she has a foal by her side; and a mare becoming anestrus (shutting down) as a result of hormonal manipulation or lactation, secondary or diestral ovulations, early embryonic death, and prolonged or retained corpus lutea. Therefore, failure to return to estrus alone is not a reliable method to detect pregnancy.
Pregnancy determination by rectal palpation is the most common and economic diagnostic technique. In early pregnancy (day 10 to 20), the embryonic vesicle is not palpable and should not be confused with endometrial cysts. However, uterine and cervical tone is increased, compared with that in diestrus. The uterus becomes more tubular and easily defined, and the cervix is palpable as a long, thin ropelike structure. As the embryonic vesicle grows and fixation occurs, a small ventral fluctuation is palpable at the base of either uterine horn (day 21). At this stage the vesicle is 30 to 40 mm in diameter. As gestation progresses the fetal fluid bulge grows, measuring 40 to 50 mm in diameter (about the size of a hen’s egg) by day 30. In addition, the nonpregnant horn becomes extremely firm and tortuous, often making it hard to differentiate between the bulge of the pregnancy and the curvature of the nonpregnant horn. Care must be taken in postpartum mares not to confuse the still enlarged and doughy formerly pregnant horn with the fluid turgidity of the new pregnancy. At day 40 of gestation the uterus does not have as much tone and the fetal-fluid bulge is about 65 mm in diameter.
By day 50 the fetal fluid starts to extend into the nonpregnant horn, and by day 60 it extends into the uterine body. Between 75 and 100 days, the weight of the enlarging fetus and the accompanying amniotic and allantoic fluids pull the uterus cranioventrally, allowing it to rest on the ventral body wall. At this time rectal palpation reveals a fluid-filled uterus and elongated tight cervix. The fetus may or may not be able to be detected via ballotment. Between 90 and 120 days the fetus comes back into reach, and ballottement and size determination are possible. Recognition of fetal movement signifies viability; however, the fetus can be quiet for extended periods, and additional testing should be done if the fetal status is questionable. Palpation diagnosis, although reliable after 30 days, is limited by the experience and skill of the examiner, uterine tone, and small size of the conceptus before 18 days after ovulation and is often ineffective for detection of twins, especially if they are unilateral. Therefore, other techniques should be used in conjunction with rectal palpation for the earliest possible definitive pregnancy diagnosis.
Examination of the cervix per vaginam has been advocated in the past as an adjunct procedure to corroborate a pregnancy diagnosis. However, disruption of the cervical and vestibulovaginal seal of pregnancy is not recommended unless examination is necessary. When observed through a speculum, a pale, elongated cervix that is tight and positioned high in the vaginal vault is suggestive of pregnancy.
The most accurate method for diagnosis of pregnancy status in the mare is transrectal ultrasonography. Originally, this technique was practical in three situations: early pregnancy diagnosis, early confirmation of nonpregnant mares, and detection of more than one blastocyst. As ultrasonography has become more common and the attendant skills have been refined, it is routinely used to investigate uterine and ovarian health, and further applications are still being developed. These newer uses include monitoring of embryonic and fetal development and growth, fetal viability, placental function, and health and fetal sexing.
Recent advances in the quality of the ultrasound equipment have established a usefulness of small, portable machines that have high clarity and definition. With these machines, pregnancy diagnosis can be made as early as 10 days after ovulation. To perform the technique, the user must be proficient at rectal palpation. A 5-MHz or 7.5-MHz linear array or sector transducer allows imaging of ovaries, uterine horns, uterine body, and cervix (Figure 179-1). Examination of the entire reproductive tract is imperative when ultrasonography is being performed for pregnancy diagnosis because embryonic vesicles migrate to the tips of the uterine horns and to a position immediately cranial to the cervix. Observation of the diestral hyperechogenic line in the uterine body signifies the endometrial mucosal apposition confirming that the uterine lumen is being imaged appropriately (Figure 179-2). The uterine horns are generally imaged in cross section, allowing the entire horn to the ovary to be viewed for pregnancy (Figure 179-3