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 easily be incorporated into most breeding programs. Here are some examples of how sonographic diagnosis of fetal gender can be used:

  • Many breeders want to know fetal gender to help them decide whether to keep or sell a mare or her offspring. If an older mare that has produced several stakes winners is going to have a filly, they may keep her. However, if she is going to have a colt, they may sell her.
  • A weanling out of a particular mare may be sold if the gender of the mare’s next foal is known.
  • Mating lists for the coming year may 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, he can book his mare to another sire. However, if she’s carrying a colt, he can book her back to the same sire.
  • If an owner wants a Canadian-bred filly or a New York-bred colt, she can send the mare carrying the proper gender to that location for foaling.
  • Occasionally a mare that palpates as a normal sixty-day pregnancy may be carrying a dead or dying fetus. An added benefit of determining fetal gender 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.
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 will eventually 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 sonographically around 90-95 days.

Between 90-150 days of pregnancy the fetus has grown considerably and its gender can be diagnosed only about 80% of the time. At this stage the veterinarian is scanning for external genitalia (penis, mammary gland, clitoris, etc.) which are frequently 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 rear area of the fetus.

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 to 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.

Richard D. Holder, DVM, AAEP Member, graduated from the University of Texas in 1969 and Texas A & M University College of Veterinary Medicine in 1972. He has been an equine practitioner at Hagyard-Davidson-McGee Associates, PSC, in Lexington, Kentucky, since 1974 with emphasis on reproduction, and 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 between 55 and 150 days of gestation and has given numerous presentations on this subject. Dr. Holder is the owner and resides at Offutt-Cole Farm in Midway, Kentucky.

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