At midterm gestation, clinicians can benefit from a much larger gestational window to determining fetal sex compared to the traditional 10-day span near the start of pregnancy. | Photo: iStock

Knowing whether your broodmare is expecting a colt or a filly can be fun for the curious hobby-breeder and even beneficial for professional breeders as they manage their business and plan their strategies. But sexing an equine fetus usually requires catching a very short span of gestational days in which the sex differences are visible via ultrasound. It can also mean sedating the mare, clipping her, and getting her in the hands of a sexing specialist.

Fortunately, though, researchers have confirmed that, by following a few recently described steps, veterinarians can sex fetuses over a much longer period and without clipping or sedating mares, scientists in Belgium said.

Traditionally, veterinarians have been restricted to determining fetal sex 60 to 70 days after ovulation; at this point, clinicians can typically identify an embryonic sex structure called the genital tubercle—which has a different position in female fetuses than in male fetuses—via transrectal ultrasound. After that period, the tubercle is more difficult to visualize due to the volume of the fetal fluids compared to the small fetus, said Margot van de Velde, PhD, of the Ghent University Faculty of Veterinary Medicine, in Merelbeke, Belgium