Two for One: Twinning in Horses By Kentucky Equine Research Staff · April 11, 2014
Everybody likes getting something for free; that’s why “two-fer” and BOGO offers are so popular. But more isn’t always better, and a case where two can be a problem is when mares conceive twins. While animals of many species routinely give birth to multiple healthy offspring from one pregnancy, horses are not designed to nourish two fetuses and produce viable twin foals. Double pregnancies put the mare and both foals at risk, and good outcomes are rare.
Usually, mares produce one ovum from a ripened follicle every 21 days or so throughout their breeding season. If the ovum is fertilized by a stallion’s sperm, the mare becomes pregnant. Some mares produce two follicles and release two ova during a cycle, and if both are fertilized, a twin pregnancy results. Some mares including those of Quarter Horse breeding apparently double-ovulate only rarely, while the phenomenon is far more common in other horses such as Thoroughbreds and some Warmblood breeds. Mares are most likely to produce more than one ovum if they are not nursing a foal; have not produced a foal in the previous season; have had a history of twin pregnancies; and are young, healthy, and at the peak of their fertile years.
Twins are sometimes carried to term and born healthy with no injury to either the foals or the mare, but this is the exception. Mares that conceive twins often lose one or both embryos within the first weeks after breeding, and these mares will usually come back into season later in the spring. Some mares will carry twins for several months before aborting halfway through the pregnancy, thereby losing a year of productivity. If both fetuses survive until term, there is an excellent chance that the mare will have a difficulty delivery that can end in the death of one, two, or all three horses. When twins are born alive, one or both may be undersized and weak, with reduced potential of maturing into productive animals. As with any sick foal, twins often need intensive around-the-clock care. If the mare was injured during the delivery, she will also need care including treatment for a retained placenta that is common after twin births.
All in all, twin foals are no bargain, and most breeders will ask the veterinarian to reduce the pregnancy to a single embryo if twins are detected during an examination conducted 14 to 16 days after the mare has been bred. At this time, the veterinarian can crush one of the embryos, usually the smaller one, allowing the mare to have a normal pregnancy. If twins are detected only in later months, other methods can be used to remove a growing fetus, though these procedures are more difficult and may cause the mare to abort the other fetus. An early post-breeding exam is important in dealing with twin pregnancies, usually allowing the mare and single foal the best chance for a successful outcome.