Why is it bad for horses to have twins?

Answer 1

While horses can successfully have viable twin foals the actual success rate is quite low. Typcially, a mare that has produced twins will lose one at an earily stage of development and the breeder will be unaware of the fact that the pregnancy was ever

one involving twins.

Most commercial breeders perform ultrasounds at 14-16 days post breeding to determine if a mare has twins and the smaller conceptus is crushed to insure the mare will carry one foal to term.

Many twin pregnancies will reduce to a singleton pregnancy when the two conceptuses implant close together.

Twins that survive much past the early embryonic stages can develop several ways.

1) The twins share an equal amount of uterine space and have placentas that are approximately the same size. Each placenta has a non-villis side that has no attachment to the uterus and this decreases the nutrient intake of both foals.

While these types of twins may survive to birth, they will often be smaller, premature or (if full term) dismature. One or both may require neonatal care for several days to weeks. Treatment may be quite expensive.

2) The twins have unequal size placentas. This usually results in one fetus "starving off" when it's large enough that the small placenta cannot provide adequate nutrition.

One fetus dies and either mummifies or decomposes. Decomposition of one fetus usually causes the death of the other fetus and subsequent abortion of the mare. Mummification of the smaller foal can result in a viable foal.

3) The twins are identical and share a single placenta. Foals of this type are extremely rare and both are smaller than a normal singleton foal. While foals may be born alive they may have many of the same deficits and often the smaller foal has

considerable health issues. One or more may require considerable veterinary care.

Twin births often result in dystocia (abnormal foaling position) which may require a veterinarian at the foaling or surgical intervention, resulting in the loss of the mare, one or more foals or all three.

A large mare may be able to carry twins to full term but they may be too small to

nurse their own dam and the mare may need to be milked and the foals fed until they are large enough to nurse on their own.

Last but not least, the mare may need a full breeding to recover as not only the production of twins but their subsequent nutritional needs will affect her ability to get back in foal.

Based on the low probability that a mare can carry twins to term, or potential economic cost or emotional cost of a lost, mare, foal, foals or all three there is

no reason to intentionally allow a twin pregnancy to continue if it can be avoided.