Monitoring High-Risk Pregnancy in Mares

The most important decision that the veterinarian must make when dealing with high-risk pregnancy in mares is when to intervene. Almost always, the foal is better off staying in the uterus no matter how hostile the intrauterine environment. The placenta of mares is quite different from that of humans and amniocentesis carries a much higher risk of abortion in mares and has not been shown to be a valuable technique to assess equine pregnancies.

Fetal ultrasound to measure heart rate and combined thickness of the uterus and placenta is the most commonly used diagnostic to monitor at-risk pregnancies. The placenta can appear thickened and may be detached from the uterus in cases of placentitis. As a very general rule, fetal heart rate runs from 60-120 beats per minute during the last month of gestation. Fetal heart rate should increase with fetal movements. Transrectal ultrasound examination (as is performed to look at the ovaries and uterus before breeding) is more reliable than scanning through the abdomen to diagnose placentitis because most cases begin at the cervix.

Periodic measurement of milk electrolytes is useful to try to predict when a mare will foal. Unfortunately, all tests to predict foaling that are currently available are much better to predict when a mare will not foal rather than when she will foal. Near foaling, milk sodium concentration is very low, potassium is very high, and calcium increases the most nearest foaling. Placentitis can affect milk electrolytes, but they are a very useful and mostly reliable method to determine when to be most observant of pregnant mares for foaling.