The schedule for determination of pregnancy varies among breeding farms. One schedule is as follows: 1) days 14–18—check for twins; if open, mare can be rebred on days 19–20; 2) days 25–30—evaluate normal embryo development (heartbeat present at 24–25 days), recheck for twins; 3) days 40–60—evaluate normal fetal development; 4) fall check—confirm mare is still pregnant.

Palpation Per Rectum

The pregnant cervix should be tightly closed and elongated with a prominent portio vaginalis 14–21 days after ovulation. Uterine tone increases and the uterine wall thickens so that by 14–18 days, the endometrial folds can no longer be readily palpated.

The conceptus develops in a recognizable pattern of size and shape, allowing estimation of age based on palpable characteristics. In maiden and barren mares, at 25–28 days of gestation, a careful, experienced palpator may be able to feel the embryonic vesicle ventrally at the base of one uterine horn, producing a bulge 3.5 cm in diameter. At 30 days, the uterine horns are small with pronounced 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 is thin over the expanding conceptus. At 42–45 days, the conceptus occupies about half of the gravid uterine horn and is 5–7 cm in diameter. By 48–50 days, the enlargement of the conceptus begins to involve the uterine body and is 6–8 cm in diameter and 8–10 cm long.

At 60 days, nearly the entire gravid horn and half of the uterine body are filled with conceptus, but the nongravid horn remains small with considerable tone. The 60-day conceptus is 8–10 cm in diameter and 12–15 cm long. After 85 days, the turgidity of the conceptus decreases such that the fetus becomes palpable. At 90 days, 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 100–120 days of gestation, the gravid uterus is positioned cranial to the pelvic brim in the abdominal cavity. The ovaries are positioned cranially and ventrally and closer together because of the downward traction exerted by the enlarging uterus on the broad ligament. After 150 days of gestation, the ovaries are not routinely felt per rectum. As the conceptus/gravid uterus enlarges, its dorsal surface comes back into reach. One should always confirm that two uterine horns with palpable endometrial folds cannot be identified in the pelvic canal before making a midterm pregnancy diagnosis.

Ultrasonography

The spherical shape of the equine embryo and the characteristic pattern of the development of the fetal membranes permit accurate estimation of stage of gestation by ultrasonography until 45 days after ovulation. The embryo may first be imaged in the uterus with a 5.0-MHz linear transducer at 9–10 days as a round nonechogenic yolk sac 4 mm in diameter. The anechoic, spherical conceptus moves throughout the lumen of the uterine horns as well as into the uterine body from day 6–16. Due to specular reflections, the early conceptus is seen to have a bright white (echogenic) line on the dorsal and sometimes the ventral aspect of its image. This specular reflection, embryonic motility, and linear growth rate may be helpful in differentiating an early <16 day, motile embryo from some uterine cysts.

At day 17–18, the conceptus has a characteristic “guitar-pick” shape. At day 21, the embryo proper can be seen in the ventral aspect of the yolk sac. At day 25, a heartbeat should be present in the embryo proper, and the allantoic fluid is imaged as an anechoic compartment ventral to the embryo proper. As the allantoic cavity enlarges, the yolk sac comprises a decreasing portion of the conceptus. The position of the allantois and relative sizes of the developing allantoic cavity and the regressing yolk sac can indicate the stage of gestation between days 25 and 45. At 30 days, the allantoic cavity occupies approximately one half of the conceptus, so that the size of the yolk sac is the same as the size of the allantoic cavity. At 45 days, the only visible fluid cavity is the allantoic cavity, and the fetus appears to be suspended from the dorsal wall of the uterus by its umbilical cord and is positioned in dorsal recumbency.

Endocrine Tests

Cells from the conceptus invade the endometrium to form endometrial cups that produce equine chorionic gonadotropin (eCG, formerly called pregnant mare serum gonadotropin) between 40–120 days of gestation. Increased serum concentrations of eCG 40–120 days after ovulation are indicative of the presence of endometrial cups. Concentrations of eCG may remain elevated until 120 days, even if fetal death occurs (false positive). A false negative eCG pregnancy test can result if blood sampling is done in a pregnant mare before 40 or after 120 days.

Estrone sulfate is produced by the fetus and is a good indicator of fetal viability. Plasma and urine concentrations of estrone sulfate are elevated after 60 days and 150 days, respectively, of pregnancy.

Last full review/revision July 2011 by Patricia L. Sertich, MS, VMD, DACT