Fetal Sex Determination in the Mare Between 90 and 150 Days

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 technique is required to determine fetal sex using transrectal ultrasonography. The examination involves locating the gonads and the external genitalia of the fetus in the male that includes the penis, glans penis, and prepuce. Identifiable in the female are the gonads, mammary glands, teats, and clitoris. With a single examination performed at 90 to 150 days of gestation, the skilled veterinarian can determine fetal sex on initial exam in 90% of cases with 99% accuracy.

Around 150 days of gestation, the fetus begins to assume an anterior presentation. In this position, the fetal head is easily accessible, while the pelvic area is beyond the veterinarian's reach. Between 150 to 200 days of gestation, determination of fetal sex using transrectal ultrasonographiy is difficult, and the likelihood of making an accurate diagnosis falls to 10-25%.

At that stage of gestation, a transabdominal (viewing through the belly) ultrasonographic examination typically is required to identify the external fetal genitalia. For greater ease and accuracy of fetal sex determination, Holder recommended the use of transrectal ultrasonographic techniques earlier in gestation.

About the Author

Rallie McAllister, MD

Rallie McAllister, MD, grew up on a horse farm in Tennessee, and has raised and trained horses all of her life. She now lives in Lexington, Ky., on a horse farm with her husband and three sons. In addition to her practice of emergency and corporate medicine, she is a syndicated columnist (Your Health by Dr. Rallie McAllister), and the author of four health-realted books, including Riding For Life, published by Eclipse Press and available at www.ExclusivelyEquine.com or by calling 800/582-5604.

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