Sexing the unborn foal

Since its introduction in the early 1980s, the use of ultrasound scanning has increased considerably and had an enormous impact on horse breeding.

Ultrasound was originally used only for pregnancy diagnosis but it is now an integral part of the management of mares at all stages of their reproductive cycle. Its usefulness for predicting optimum breeding time and as a diagnostic tool for problem breeding mares has been established, while the latest application of ultrasound is in determining the sex of the foal.

The first reports on sexing the equine foetus using ultrasound appeared in the late 1980s, but the technique is still not widely used in Britain.

From day 55 of pregnancy, the developing foetus can be identified as a “miniature foal”, with several features easily recognised.

The technique of foetal sexing is based on the assessment of a structure called the genital tubercle. This is the embryonic structure that will develop into the clitoris in the female and the penis in the male. From around day 60 of pregnancy, male and female genital tubercles can be reliably detected using a good quality ultrasound machine.

Both male and female genital tubercles appear similar in appearance and the key to diagnosis of the sex is their location: the female genital tubercle is found under the tail, whereas the male genital tubercle is located just behind the umbilicus.

The optimum time for diagnosis of foetal gender would appear to be around day 60 to 70 of gestation. The genital tubercle begins to migrate by day 55 and is usually fully migrated by about day 60.

After day 75 of pregnancy, the position of the foetus in the uterus prevents the ultrasound probe from being able to scan the necessary area to confirm the sex of the growing foal. However, sexing is possible later in pregnancy by scanning through the mare’s body wall.

A considerable degree of expertise is required to become proficient in the technique of gender determination, and in the hands of an experienced operator, the technique is accurate and rapid, although costly.

It should be noted that while this technique allows the identification of the sex of a foal, it cannot control or influence the gender of the growing foetus.

This article was extracted from a Horse & Hound veterinary clinic feature (29 May 03), which also covered the ultrascanning process in detail, plus the pros and cons of sexing the unborn foal. To purchase a back issue of this magazine contact (tel: 020 8532 3628).