The outcome of twin pregnancies in mares.

Brood-farm veterinarians were surveyed and theriogenology records were examined to obtain information on the natural outcome of rectally-diagnosed twin pregnancies. Four of the veterinarians estimated that 50% of mares with twin embryos had single foals. In the analyses of palpation records, 31 53 (58%) of the diagnosed twin pregnancies terminated in birth of one foal. However, the loss of one embryo without the loss of the other occurred significantly more often before day 40-42 (30 53 ; 57%) than after day 40-42 (1 16 ; 6%). Of the remaining 15 mares with diagnosed twins at day 40-42, 5 had two foals and 10 had no foals. The methods used for intervention when twins were diagnosed were unsatisfactory. Complete termination of pregnancy with a prostaglandin or an intrauterine flushing resulted in failure to reestablish a singleton pregnancy during the operational breeding season in 10 11 mares. Attempts to eliminate one embryo resulted in loss of both in 6 7 mares. The results indicated that, at the present time, nonintervention should be given more consideration as a method of handling twin pregnancies.