Twin foals derived from the fertilization of one egg by one sperm (the scientific term is monozygotic twins) after embryo transfer have been reported in the past, said Semira S. Mancill, DVM, a resident in Large Animal Medicine & Surgery at Texas A&M University, during a presentation at the 2008 American Association of Equine Practitioners convention, held Dec. 6-10 in San Diego, Calif. But veterinarians had never performed DNA testing to confirm such a circumstance.

That all changed at Texas A&M. It started with a 9-year-old Thoroughbred mare giving birth to stillborn foals 10.5 months into her pregnancy. Both foals were bay fillies. The mare had received a single embryo via embryo transfer.

Tissues from both the recipient and donor mare, as well as from the two stillborn foals, were submitted for microsatellite analysis. The analysis verified that the foals had identical genotypes and resulted from the transferred embryo.

Parentage testing revealed that the embryo donor qualified as a possible dam and that the foals could not have resulted from conception of an ovum from the recipient mare.

"To our knowledge," Mancill told the group, "this is the first report of monozygotic twin pregnancy after embryo transfer verified by genetic analysis."