“Aww, how cute! She’s going to have twins!” Stop right there! Twinning in horses is not cute, it is not nice; in fact, it is truly a terrible thing.

When a horse is in foal, the placenta covers the entire lining of the uterus. This is very important as the foal requires a huge amount of nutrients to develop. When a mare is in foal with twins, the two placentas must share the surface of the uterus. Because of this, neither foal gets enough nutrients to develop properly.

Sometimes one embryo dies early in the pregnancy, allowing the other to grow, and possibly to develop fully. Often the dead embryo is mummified inside, and is born with the live foal. Sadly, not all live foals survive, despite the death of their twin.

More often, both embryos die before term, and the mare aborts the pregnancy. This often happens quite late in the pregnancy, resulting in a sad discovery for the owner. Other times it happens earlier, and the mare owner does not discover the loss until the mare is due, and there is no longer a foal.

In very rare cases, both foals may be carried to term. Sadly, the survival rate for twin foals is very low. Often one is under developed and dies shortly after birth. Other times, the mare will reject one foal, but care for the other. The rejected foal must then be raised by hand, or it will die.

Often the live foals do not survive despite the best efforts of the owners. The immune system may be weak, or there may be developmental problems. While you might expect the larger foal to be the stronger one, this is not always the case, and both babies are at equal risk.

Another risk to consider is that mares can be seriously damaged by carrying a dead foal to term, or in the foaling process. The uterus can be scarred, destroying the mare’s fertility, and the extra set of limbs during foaling can be a real hazard, increasing the risk of tearing or poor positioning.

So, what can you do about it?

The best thing is to prevent twin pregnancies in the first place. After breeding your mare, have her ultrasounded before she reaches 30 days pregnancy. The ultrasound will show whether there is one foal or two. If there are two foals, your vet can sometimes “pinch” one embryo, killing it so that the other can survive. Sometimes you can cut your mare’s feed back so that her body believes that she is “starving” and thus aborts one of the embryos.

Interestingly, 80% or more of twin pregnancies resolve themselves. Unfortunately, you can’t count on this. If your mare is still carrying twins at 30 days, it is necessary to abort the pregnancy. This can be done by administering a “hot shot” of progesterone that causes the mare to slip the pregnancy and come back into season. The advantage of aborting the pregnancy is that you can usually get another breeding out of the mare that season, and hopefully end up with a single pregnancy. Waiting longer that 30 days prevents the mare from cycling again, making it impossible to rebreed during that breeding season.

Twins are never worth the risk. Responsible breeders ultrasound their mares to prevent twinning whenever possible. However, it is possible to miss a twin pregnancy despite careful measures. If this happens, all you can do is take care of your mare and any surviving foals and hope for the best. In very rare cases one or both foals do survive, and become productive horses.