Reduction of Twin Pregnancy More Risky for Older Mares

It’s not uncommon for mares to conceive twins, but the chances of successfully carrying both fetuses to term and delivering two healthy foals are not good. Even when twin foals are born alive, at least one is likely to be much smaller than normal. Twins may be underdeveloped, weak, and prone to an array of health problems requiring intensive care that many owners do not have the time or skills to provide. Those that survive have a less-than-average chance of maturing into full-sized, usable horses, though some twins do manage to completely overcome their early challenges.

Because twin pregnancies often have poor outlooks, veterinarians usually manually terminate one embryo if they detect multiples when a mare is checked by ultrasound to confirm pregnancy. A study conducted at Rood and Riddle Equine Hospital in Lexington, Kentucky, examined the results of twin reduction in 129 mares. The study also looked at the records of 127 comparable mares with single-embryo pregnancies.

The researchers noted the ages of all mares, location of embryos, month of breeding, and other variables and compared these factors to the outcomes of the pregnancies. Results showed that when all factors were considered, the only one that was significantly related to pregnancy loss after embryo reduction was age of the mare. Mares over the age of nine were more likely than younger mares to lose the entire pregnancy after reduction of one embryo.