Filly or Colt? Find Out Before It's Born

Knowing the gender of a foal before it is born helps the horse owner or breeder better manage several different aspects of the horse business. This new management tool is very safe, accurate, and can be incorporated into most breeding programs quite easily. Here are some examples of how ultrasound diagnosis of fetal gender can be used:

  • Many breeders want to know the gender of the unborn foal to help them decide whether or not to keep a mare or her current offspring. For example, if a breeder knows that the mare is going to have a filly, he or she might keep the mare. However, if she is going to have a colt, the owner might sell that mare and keep another one. Also, a weanling out of a particular mare might be sold if the gender of the mare's next foal is known. For example, if the breeder wants a filly to carry on the line, and both the weanling and the fetus are fillies, he can sell the weanling. This cuts his costs and gives him some income.
  • Mating lists for the coming year might depend on the gender of the foal being carried. Perhaps an owner wants a filly by a particular sire. If his mare is already carrying a filly by the desired sire, he can book his mare to another sire. However, if she's carrying a colt by the desired sire, he might book her back to the same sire to try for a filly.
  • If an owner wants a Canadian-bred filly or a New York-bred colt, he can send the mare carrying the proper gender to that location for foaling.
  • Occasionally a mare which palpates as a normal sixty-day pregnancy might be carrying a dead or dying fetus. An added benefit of determining fetal gender by ultrasound is verifying the mare is carrying a single live fetus.

It is important to understand that this new tool identifies the gender of a fetus only and cannot control whether it is a male or female.

How It's Done

Your veterinarian begins the process by scanning for a live normal fetus. The fetus is examined for gender by locating a structure called the genital tubercle. The genital tubercle (which eventually will become the penis in a colt and the clitoris in a filly) develops on the midline of the fetus, between the hind legs. At around 55 days, the structure moves toward the umbilicus in a colt and toward the anus in a filly. Therefore, the gender of a fetus cannot accurately be determined before about Day 60, when it has fully migrated.

After 75 days, the uterus is carried over the pelvic rim by the fluid of the pregnancy and the fetus moves to the lowest part of the uterus, making the rectal ultrasound approach difficult. However, as the pregnancy progresses, the fetus grows and extends back toward the pelvis, allowing it to be viewed again ultrasonographically around 90-95 days.

Between 90 and 150 days of pregnancy, the fetus grows considerably and its gender can be diagnosed only about 80% of the time. At that stage, the veterinarian is scanning for external genitalia (penis, mammary gland, clitoris, etc.) that frequently are difficult to see because they are not well developed until around 110 days. After 150 days, the fetus has grown so large that it becomes even more difficult to view the area of the fetus needed to determine gender.

The ability to determine the gender of a foal before it is born requires planning. Even the best equipment and the most experienced veterinarian will be faster and more accurate (a 99% accuracy rate can be attained) when the mare is between 60-75 days pregnant. Before and after this time, size, positioning, and other factors make fetal gender determination difficult. Since timing is critical, tell your veterinarian ahead of time that you would like to know the gender of the fetus so that the service can be arranged.

About the Author

Richard D. Holder, DVM

Richard D. Holder, DVM, graduated from the University of Texas in 1969 and Texas A&M College of Veterinary Medicine in 1972. He has been an equine practitioner at Hagyard-Davidson-McGee Associates, PSC, in Lexington, Ky., since 1974 with emphasis on reproduction, and he currently holds the office of treasurer for the firm. In the past several years, he has been instrumental in developing the technique for equine ultrasonic fetal sex determination and has given numerous presentations on this subject. Holder is the resident owner of Offutt-Cole Farm in Midway, Ky.

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