Twinning in mares is the leading noninfectious cause of pregnancy loss. The vast majority of twin fetuses are aborted in mid to late pregnancy. The mare can suffer devastating consequences as well – such as uterine scarring as a result of the abortion, or retained placenta, which can be life-threatening or permanently impair fertility. Abortion is thought to occur because the uterus is unable to sustain the development of two fetuses at the same time. In a small number of cases, one or both foals survive. They are usually very small and weak, and require extensive medical care. The incidence of twinning varies with the size of the mare – the larger she is, the more likely she is to twin. Twin pregnancy is practically unheard of in ponies, while the incidence is five percent or more in thoroughbreds and draft horses.
The best way to avoid the devastating consequences of twinning is to detect all twin pregnancies very early in gestation and to eliminate one of the embryos. Pregnancy can be detected as early as 10 days gestation with quality ultrasound equipment. An experienced equine veterinarian can detect the presence of twins and eliminate one of the embryos by manually applying pressure to it. The cost is reasonable and the chances of success are very high. A successful outcome is the most likely if this procedure is performed by day 15 after ovulation. When performed later, in some instances the other embryo can be injured and lost during the procedure.
Mares pregnant with twins 35 days or more postovulation are difficult to manage. Sometimes nature will eliminate one of the embryos naturally, but this is unlikely once the pregnancy has progressed to this point. Most veterinarians recommend termination of the entire pregnancy at this stage. There is an additional drawback – hormonal changes this late in pregnancy may very well prevent successful rebreeding for months – the entire season may be lost for that mare.
Veterinarians often get requests to examine mares for the presence of twins at 60 days of gestation or even later. It is impossible to manually palpate a mare at this stage or later and detect twins. In addition, ultrasound with standard equipment used by the majority of veterinary practices will not accurately diagnose twins at this stage. Breeders have two choices in this situation – wait and see what happens or go to a clinic with special ultrasound equipment – at the present time this is usually only available at the largest clinics or a university. Since the only practical option at this stage is to abort the pregnancy, it is definitely more desirable to avoid this dilemma with a very early exam – preferably by day 15 postovulation.