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AskHQ: Double trouble

Q: How often do horses have twins?

A: Twins are conceived as a result of the stallion’s sperm fertilising two eggs released when a mare double ovulates. Thoroughbreds and Warmbloods are the most likely to have mutliple ovulations, and they therefore tend to have more twins than other breeds.

However, nature does not favour the birth of equine twins, and many mares abort twin pregnancies within the first 40 days. Of the mares who carry twins longer than six weeks, 80% will abort during the eighth month of pregnancy. Aborting so late in the pregnancy can cause all sorts of complications for the mare, such as trauma, illness, infection and reduced fertility for the next breeding season. While it is difficult to pinpoint exactly how often mare’s have twins, only 1 in 100 twin pregnancies goes all the way to full-term. The reason is that the mare’s uterus is not designed to support two babies at once.

If the mare does deliver one or two live foals, the foals are likely to have problems of their own. Twins are usually born prematurely, and are below average in size as their combined weight will total somewhere around the weight of a normal, single foal. They rarely catch up to normal size and weight.

Veterinarians will therefore commonly recommend removing one of the twin embryos early in the pregnancy to maximise survival chances and well-being of both the mare and remaining foal.