Pregnancy Diagnosis in Mares
Breeding your mare can be an exciting way of expanding your herd. A lot of preparatory work goes into choosing the stallion, managing the mare through her cycle, and obtaining the semen for the time of ovulation. However, the work is not done after the mare has been inseminated! While it is a lot of fun to see the your mare’s embryo on ultrasound, more importantly, proper pregnancy diagnosis can lead to a safer and more cost-effective breeding season for your mare.
In most circumstances, your veterinarian will recommend an ultrasound for pregnancy detection at days 14, 25, and 35 post-ovulation. Each one of these checks is performed for a specific reason. Additional checks may be necessary for particular cases, and your veterinarian will discuss these with you should your mare require them.
Day 14: While an equine embryo can be detected as early as day 11 on ultrasound, there is generally little use in checking the mare at this stage. Why? In the case of a twin conception, the embryos can be different ages depending on when the follicles were ovulated. So even if an embryo is seen at day 11, your veterinarian will need to come back and check the mare in a few days to make sure that a younger twin hasn’t appeared that was too small to see at day 11. The reason why the check should be done by day 14-15 is that should a twin be detected, reduction of one twin is easiest at this early stage of gestation (the difficulty of the procedure increases after day 17). Twin pregnancies are rarely successful in mares, and usually result in abortion of both fetuses or a dangerously difficult foaling. The chance of having a mare deliver two healthy fetuses at term is estimated to be 1 in 300 000, so your veterinarian will advise reducing one twin in the case of a twin conception.
Day 25: This is the earliest that you can see the embryo’s heartbeat. An embryo can only be determined to be viable (alive) if a normal heartbeat is seen on ultrasound. This is important information, because if the embryo is not viable, the mare should be returned to heat for rebreeding. Also, if you are planning to sell the mare, you may not legally claim that she is confirmed in foal until a heartbeat is seen on her embryo.
Day 35: At this stage, the mare is checked again to ensure the developing embryo is still viable. In the days following this check, if the embryo is lost, the mare will not return to heat as special hormonal changes for pregnancy have taken place, preventing return to cyclicity. If the embryo is not viable at day 35, the mare can be induced to abort the conceptus so that she can be rebred before these changes start to take effect. Failure to do so may result in an inability to successfully rebreed the mare in the given season.