determining equine fetal sex

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.

The 10-day window also tends to fall right during the busy breeding season, making it an inconvenient time for breeders, van de Velde said. Additionally, the fetus at that age is usually very active, rarely holding still long enough for clinicians to get an easy view.

At midterm gestation, however, clinicians can benefit from a much larger gestational window, she said. The mares can undergo ultrasound scan anytime between 120 and 210 days of gestation if clinicians follow a few basic but important techniques.

First, veterinarians must use a good-quality ultrasound machine, van de Velde said. Most of the “current generation” ultrasound machines are sufficient for reliable results, even in a farm setting, she said.

Second, the veterinarians must gain experience, learning to recognize various fetal sex characteristics at that age of gestation. “It’s not so hard to get a good level of accuracy,” she said. “It’s just that it takes more time for an examination when you are not that experienced. The more cases you see, the easier you recognize the different features belonging to a filly or a colt. And the more cases you do yourself, the faster you become in getting nice views and in making your diagnoses.”

Van de Velde and colleagues examined 121 mares in mid to late gestation (120 to 270 days). They performed transrectal ultrasounds on the mares up to about Day 150 and transabdominal ultrasound on mares in later pregnancy (more than 150 days). That’s because the fetus moves out of the rectum-based viewing range at about 150 days, she said.

At more than 210 days, the fetal reproductive organs were difficult to view regardless of the positioning of the ultrasound equipment, van de Velde. Even so, their research group succeeded in finding at least one sex characteristic in the fetuses up to 257 days of gestational age.

For transabdominal ultrasound, the researchers performed ultrasound at the farm on unsedated, unclipped mares. Alcohol was sufficient to get good contact with the skin, she said.

The key is to look for two or ideally three specific sex characteristics that can be seen at that age, she said. In males, the easiest traits to identify are the penis and a line that runs inside the testicles. In females, the most obvious ones are the udder and the vascular ring in the ovaries. One thing not to look for is the size, shape, or location of the gonads (fetal ovaries and testicles), because these traits aren’t distinguishable between males and females at that age.

In their trials, the researchers achieved an accuracy rate of 98% in the mares of 257 days gestation or less, said van de Velde. In the 2% they got wrong, they’d only seen one sex characteristic, which wasn’t enough to make a definite diagnosis.

Any equine veterinarian can achieve a good accuracy rate in sexing midterm fetuses in the field, with training and practice, she said.

“In veterinary medicine there are still a lot of ‘general’ practitioners who are doing basic orthopedics, internal medicine, and reproduction,” said van de Velde. “They often don’t have or take the time to learn ultrasound properly. But I think veterinary medicine is developing in a similar way to human medicine, and every veterinarian that is working as an equine reproduction specialist should be able to diagnose fetal gender.”

The study, “Equine foetal gender determination in mid‐ to late gestational mares: A practical inquiry,” was published in Reproduction in Domestic Animals