The authors concluded that veterinarians should not only examine mares 13 to 16 days post-ovulation to hunt embryos, but also examine the ovaries to see if more than one ovulation occurred. | Amy Katherine Dragoo

“Although rare in many breeds, multiple ovulations, mostly double ovulations, can occur in at least 25% of estrous cycles in Thoroughbreds and Warmbloods,” said Mina Davies Morel, PhD, a senior lecturer in animal reproduction at Aberystwyth University, in Ceredigion, U.K.

Many double ovulations are synchronous (occurring at the same time); however, asynchronous ovulation can occur in which two eggs are ovulated several days apart. Because equine sperm is known for its longevity, it can live inside the mare’s reproductive tract for several days. If a mare is bred at the time of a first ovulation but a second ovulation occurs five or six days later and that egg is also fertilized, two embryos of different ages can coexist.

“In this scenario, if a routine ultrasound examination were conducted 14 to 16 days after the first ovulation, it is possible the second embryo would be missed,” Davies Morel explained. “This is because embryos can’t usually be seen on ultrasound until 11 days after