Success rates of various techniques for reduction of twin pregnancy in mares.

Abstract

Twinning is an important cause of pregnancy loss in mares and is often associated with economic loss. Most twin pregnancies can be expected to result in early gestational death, late-term abortions, or delivery of small foals with retarded growth. These foals can be more susceptible to infections and slower to develop after birth, compared with singletons. 1,2 Abortion in late gestation and the birth of twins often involve dystocia and trauma to the reproductive tract and are associated with poor rebreeding potential. 3 Prior to 1997, reported incidence rates of twinning-associated spontaneous abortion in mares ranged from 112 of 516 (22%) 4 to 689 of 1,788 (38.5%). 5 Results of a 9-year study 6 undertaken between 1988 and 1997 in the United Kingdom have shown that the rate of spontaneous abortion in mares was as low as 75 of 1,252 (6%). Although the direct effect of increased availability of ultrasound scanning on twin-associated spontaneous abortions has not been studied, it is possible that routine early scanning practices have contributed to this reduction. A report in 1973 indicated that only 9 of 62 (15%) twin pregnancies produced 2 live foals and that foals born alive were usually emaciated. 4 In that study, 4 13 of 31 (42%) foals born of twin pregnancies had died by 7 days of age, compared with 15 of 1,665 (0.9%) singleton foals. Mares that give birth to twins are also likely to have an oversized uterus, which delays postpartum involution and has been associated with reduced live foaling rates in subsequent breeding seasons. 1,7 Undetected and unmanaged twin pregnancies have important implications, with negative effects on animal health and economic loss being the principle detriments. Reduction of twin pregnancies without subsequent loss of the remaining foal is particularly important in animals with high economic value, mares with poor subsequent breeding potential, cases where the diagnosis is made late in gestation or late in the breeding season, and situations where a stallion is not available for rebreeding. Twin births in mares can and should be avoided,