Determining the sex of an equine fetus, often referred to as “fetal sexing,” is commonly performed in broodmare practice. Horse owners may prefer a filly or a colt depending on the intended use of the foal. Ultrasonographic imaging, which is both accurate and non-invasive, allows an experienced veterinarian to determine fetal sex in horses. Equine sex can be determined prior to foaling by visualization of the developing fetal reproductive organs at specific time points in gestation.
In female and male horses, the genital tubercle is commonly located on the abdomen until approximately day 55 of gestation. By day 60 it migrates to form the penis in colts and the clitoris in fillies. Transrectal ultrasonography is the preferred approach to image the early pregnant mare. In the male fetus, the genital tubercle can be identified closer to the umbilicus where the penis will develop on the ventral abdomen. While in the female fetus, the genital tubercle migrates towards the anus where it can be identified between the hindlegs.
To confirm your findings in early pregnancy, the equine fetal gonads can easily be imaged by transabdominal ultrasound between day 120 and 200 of gestation. The male gonad, testis, has a specific echotexture where the mediastinum is very prominent centrally. The female gonad, ovary, has a homogenous echotexture but a peripheral pattern of blood flow that can be appreciated with color flow doppler. In a female equine fetus, the absence of male genitalia and the presence of the vulva or the development of the mammary gland can be observed.
Ultrasound is the most common and reliable method for equine fetal sexing. Other methods are being researched, including genetic testing. While not commonly used for routine sex determination due to cost and complexity, detection of fetal DNA in circulation and/or fetal fluids can be obtained through amniotic fluid sampling. This is more invasive and typically reserved for specific situations where precise genetic information is required.
If you’re considering determining the sex of a foal, consult with a veterinarian who has experience with equine reproductive ultrasounds. Contact us if you’re interested in scheduling an ultrasound with one of our equine reproductive veterinary specialists.