Mother Nature clearly does not favor the birth of equine twins, as a significant number of twin embryos spontaneously abort within the first six weeks of pregnancy. Of the twin conceptions present after 40 days of pregnancy, about 80% will subsequently abort, most often after the eighth month of pregnancy, according to the University of California, Davis, Center for Equine Health. With late-term abortion, the mare can experience major complications including trauma, illness, infection, inflammation of the laminae (leading to founder), and reduced fertility for the next breeding. In the rare case where the mare delivers one or two live foals, there are increased foaling problems for the mare and greater loss of life for the foals during the first two weeks of life. The combined birth weight of the twins equals the size of one normal, single foal, and the twins never catch up to normal weight and size.
With the odds stacked so unevenly against the twin pregnancy, horse owners should endeavor to remove one of the twin embryos early in pregnancy. Doing so maximizes the chances of the mare continuing on with a single, healthy pregnancy.
Not Enough Room
Twin pregnancies in the mare nearly always occur when the mare ovulates an egg from each of two ovarian follicles and both eggs are fertilized, resulting in two embryos. The ovulations might occur at the same time, or a couple of days apart. In horses, it is rare that a fertilized egg splits to form identical twins.
Twin embryos either occupy their own uterine horn (paired branchings of the uterus leading from the body of the uterus to the oviducts) or share a horn. "If two e