Accurate determination of fetal sex can have a profound impact on the broodmare management practices of horse owners and breeders. While the gender of a foal influences its value, it also influences the value of the gravid (pregnant) mare. Knowing fetal sex prior to foaling allows horse owners and breeders to make timely, informed management decisions, including those related to foaling location and subsequent mating of the mare. Demand for equine fetal sex determination among horse owners has risen significantly in the past decade, and it will likely continue to increase.

Richard Holder, DVM, of Hagyard Equine Medical Institute in Lexington, Ky., described ultrasonographic techniques used to determine fetal sex at various stages of gestation, focusing primarily on those employed between 90 and 150 days. He characterized what can be viewed at different stages of pregnancy during his presentation at the Hagyard Bluegrass Equine Symposium 2006, which was held October 18-21 in Lexington.

Using transrectal ultrasonography, the skilled veterinarian can accurately determine equine fetal sex after 54 to 55 days of gestation. Between Days 55 and 90, fetal sex determination is possible in 95% of initial exams, with 99% accuracy. At that stage of the mare’s pregnancy, the veterinarian must first locate the fetal genital tubercle, a structure that will ultimately form the penis in the male and the clitoris in the female.

At 80 to 90 days of gestation, the sex of the fetus becomes more difficult to assess due to the position of the uterus in the posterior abdomen.

“At approximately 80 days, the fluid of the pregnancy begins to pull the uterus over the rim of the pelvis into the abdominal cavity, which makes it harder to find,” explained Holder.

Later, as the uterine contents increase in size, the elevated uterus is more accessible. Between 90 and 150 days of gestation, a second techniqu