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ARTICLE: Pregnancy Determination and Pregnant Mare Care

Pregnancy determination can be done much earlier and with far greater accuracy with the diagnostic ultrasound. The old days of the 18-day “tone check” and the 32-day rectal pregnancy check are gone. Our current recommendation is to do an initial ultrasound (u/s) at 16 to 18 days post breeding (or post ovulation if that time is known). Earlier checks can be done by a day or two earlier if the mare cycles early or if coordinating with a distant stallion requires extra lead time. An additional u/s should be done at 27 days to check for a fetal heartbeat (not present until 26 or 27 days post ovulation). Additional “peace of mind” pregnancy exams can be done with u/s at 60 days (most cases of early fetal death occur from 12-55 days), and at least one additional pregnancy determination at 5 month to coincide with the first rhinopneumonitis vaccine.

Another benefit to the use of ultrasound is to rule out twin pregnancies. The initial and the “heart beat” ultrasounds are the minimum needed to rule out the vast majority of twins. If twins are discovered or if the mare has a history of twinning, additional u/s exams will be recommended. Because of the two dimensional picture provided by an u/s, a twin can hide behind its sibling. If twins are suspected and the mare is beyond 50 days gestation, the u/s would be done across the abdominal wall instead of rectally. We are also available to follow fetal health with ultrasound in later pregnancy in mares that have experienced prior pregnancy problems.

Pregnant mares should remain a part of the regular vaccination and deworming routine for horses on the same property, but some additional precautions must be taken. The newer intranasal vaccines for stangles and influenza are not approved for pregnancy, and, if protection for these diseases is required, intramuscular vaccines are used. A routine vaccination should be given 4-6 weeks pre-foaling. In addition, the pregnant mare should receive a series of rhinopneumonitis vaccines at the end of the 5th, 7th, and 9th months of pregnancy to prevent the abortion caused by the virus. Mares should be dewormed every 2-3 months (depending on their environment) but the last worming should fall 30 days prefoaling.

Feeding the pregnant mare requires a watchful eye so that the mare retains good body condition and muscle tone. Obesity will make late pregnancy uncomfortable and foaling more difficult while underweight mares have insufficient milk in early lactation and may have undersized foals. During the first 7-8 months of pregnancy, the mare should not require special feed and can usually be maintained on a diet of 50% grass or oat hay and 50% alfalfa hay. If concentrates such as grain or other pelleted food supplements are required to maintain body condition, these can be added. During the last one-third of pregnancy, the fetus will grow rapidly and additional calories may be required as well as special supplements, which can include 1) a balanced vitamin supplement, 2) a supplemental source of of calcium and phosphorus such as feed grade bone meal or other commercial supplements, and 3) a pelleted broodmare supplement which must have augmented copper levels as well as other minerals important in fetal development. This supplementation will also be a source of additional calories. Again, the objective of caloric supplementation is to keep the pregnant mare in good body condition. It will be during lactation that the new mother will really need a large increase in feed.

Exercise, in general, is good for the pregnant mare. This may only amount to a number of hours of turnout into pasture. However, if the mare was used for pleasure riding or light competition prior to being bred, these activities can usually be continued for the first 6-7 months of pregnancy. Also remember to remove the caslicks procedure (selected mares have the top of their vulva sutured closed) two weeks before the due date.

This is an exciting time for the owners of expectant mares. Please call the clinic if you have specific questions