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.