Description and genetic analysis of three sets of monozygotic twins resulting from transfers of single embryos to recipient mares.

Case Description: 3 sets of monozygotic twins resulting from transfers of single embryos to recipient mares were examined.

Clinical Findings: In all 3 recipient mares with twin pregnancies, only 1 embryonic vesicle was detected before day 25 of gestation. In 1 recipient mare, 2 apparent adjacent vesicles, each containing an embryo with a heartbeat, were visualized on ultrasonographic examination on day 37 of gestation. The other 2 recipient mares underwent ultrasonographic examination on day 30 of gestation, at which time only 1 vesicle and embryo was identified. In these latter 2 recipient mares, however, a thorough ultrasonographic examination for a second conceptus on day 30 had not been performed, as only 1 embryo had been transferred and visualized on early ultrasonographic examination.

Treatment And Outcome: All twin pregnancies resulted in death of both fetuses. Genetic analysis confirmed that each set of monozygotic twins originated from the transferred embryo.

Clinical Relevance: Monozygotic twin pregnancy may occur after embryo transfer; thus recipient mares should be examined thoroughly for multiple conceptuses, especially between 25 and 30 days of gestation. At this time, the allantoides of monozygotic twins should be visible ultrasonographically and effective management may still be possible.