Timely and accurate diagnosis is essential for successful equine breeding. Unlike many other species, mares rarely show clear external signs of early pregnancy, necessitating routine veterinary testing. Confirmation methods detect potential issues, such as twin pregnancies or early embryonic loss, before they become serious. The standard approach involves a combination of physical examinations and laboratory assays.
Early Confirmation Through Imaging and Palpation
Transrectal ultrasonography is considered the gold standard for early detection. This imaging technique allows a veterinarian to visualize the embryonic vesicle as early as 14 to 16 days after ovulation. This timeframe is chosen because the embryo is still mobile within the uterus. Confirming pregnancy at this stage is particularly important for identifying a twin gestation, a condition that rarely results in a viable outcome and requires immediate management.
The ultrasound is repeated around 25 to 30 days post-ovulation, which is the earliest time to confirm fetal viability by detecting a visible heartbeat. A strong, consistent heart rate is a more definitive sign of a healthy, progressing pregnancy than the presence of the embryonic vesicle alone. This second check is also a final opportunity to ensure that a second twin was not missed during the initial scan, as spontaneous reduction of one vesicle often occurs within the first few weeks.
As the pregnancy advances, transrectal palpation becomes a reliable, though less detailed, method. An experienced veterinarian can manually feel for subtle changes in the size, tone, and texture of the uterus beginning around 25 to 30 days. By four to five months into gestation, the fetus and enlarged uterus can be palpated more easily, though this method is unable to distinguish between a single or twin pregnancy.
Transrectal ultrasound remains the preferred method until approximately 60 to 90 days of gestation, after which the fetus naturally descends deeper into the mare’s abdominal cavity. Once this descent occurs, the fetus and placenta are examined using transabdominal ultrasound, which involves scanning across the mare’s abdomen. This later-stage imaging assesses fetal growth, checks the integrity of the placenta, and monitors for signs of late-stage complications, such as placentitis.
Confirming Pregnancy Using Hormone Assays
Confirmation also relies on measuring hormones produced by the developing pregnancy. One widely used blood test detects Equine Chorionic Gonadotropin (eCG), previously known as Pregnant Mare Serum Gonadotropin (PMSG). This hormone is produced by specialized structures called endometrial cups, which begin to form in the uterine lining around 35 to 40 days after conception.
eCG becomes detectable in the mare’s blood plasma starting around 40 to 45 days of gestation, with levels peaking near 60 days. The presence of this hormone confirms successful pregnancy and the formation of the endometrial cups, but it has a significant limitation. If the embryo dies after the cups have developed, the cups continue to produce eCG for an extended period, leading to a false-positive result regarding current fetal viability.
eCG levels generally decline and become undetectable by 100 to 150 days, necessitating a second, later-stage assay. This test measures Estrone Sulfate, a steroid produced exclusively by the combined action of a viable fetus and the placenta. Estrone Sulfate levels rise substantially after 90 to 120 days of gestation and remain high until close to foaling.
Since Estrone Sulfate requires a living fetus for its production, a positive result after the fourth month strongly indicates an ongoing, viable pregnancy. Samples for this analysis can be collected from the mare’s blood, urine, or milk. This hormonal assessment provides a reliable, non-imaging method for confirming late-stage pregnancy when the fetus is too large to be effectively scanned via the transrectal route.
Gestation Milestones and Recheck Schedules
The typical gestation period for a mare averages around 340 days, ranging from 320 to 370 days. Because early embryonic loss is common, a series of scheduled rechecks is recommended to ensure the pregnancy is progressing normally past the most vulnerable stages. This monitoring helps ensure successful management.
The first check is performed at 14 to 16 days to confirm pregnancy and address potential twin situations through selective reduction. The second check occurs around 28 to 30 days to verify the presence of a strong fetal heartbeat and confirm viability. Follow-up checks are often scheduled at 45 to 60 days to confirm the pregnancy has passed the window for most early loss and to allow for PMSG testing if desired.
A final late-stage check is recommended around 120 days to utilize the Estrone Sulfate test, confirming that the fetal-placental unit is functioning properly. If early checks confirm the mare is not pregnant, she can be quickly returned to the breeding cycle. If a mare has lost the embryo but has a persistent corpus luteum, a veterinarian may administer medication to induce her to return to heat promptly.