Equine Fetal Sex Determination

Knowledge of a fetus’ sex enables the horse owner or breeder to manage several  different aspects of his or her horse business better. For example, the value of a foal frequently is influenced by its gender, depending on its sire or dam. Therefore, we can be more accurate in assessing the value of a pregnant mare by knowing the sex of her fetus. Appraisals, selling decisions, reserves at auction, insurance coverage, mating lists, prepurchase limits, collateral limits, and foaling locations all are influenced by this information.


Most clients are having fetal sex determinations made to decide whether to keep or sell a mare. At times the decision revolves around an old mare which has had several stakes winners. If she is going to have a filly, the owner will keep her. If she is going to have a colt, the owner will sell her. An owner might sell a weanling out of a particular mare if he or she knows the sex of the foal the mare will have the following year. Sometimes an owner will race all foals of one sex and sell all of the foals of the other sex.

Sex determinations also can help owners estimate cash flow for the following year. Will the next year provide income from sales of offspring or will there be expenses from horses racing? Mating lists for the coming year, in some cases, can be dependent on the sex of the foal in utero. Perhaps an owner wants a filly by a particular sire. If the mare is carrying a filly, he or she can book the mare to another sire. If the mare is carrying a colt, the mare can be booked back to the same stallion. If an owner wants a Canadian-bred filly or a New York-bred colt, location of foaling can be predetermined.

Occasionally, a mare which palpates as a normal sixty day pregnancy might be carrying a dead or dying fetus. The fetal sexing ultrasound exam can verify that the mare is carrying a single, normal pregnancy. If a stallion has predominately more successful offspring of one sex, different stud fees for colts and fillies could be assigned and verified by a sexual determination made at 60 to 70 days.

These are just a few of the reasons horse owners are using fetal sex determination. This new management tool is safe, accurate (when done by someone skilled in the procedure), and a relatively inexpensive source of pivital information. It easily can be incorporated into any breeding program.

It should be emphasized that this new tool is for identification of fetal sex only and not for attempting to control the sex of offspring. At that stage of gestation, if the pregnancy is terminated, the mare cannot be brought back into cycle and bred that year in time for a normal foaling date the following year.

How It’s Done

Materials needed for the examination include the following: a quality ultrasound machine with a real-time linear array, five-megahertz transducer, a portable viewing stand on wheels for close eye-level viewing, fly repellent to minimize mare movement, and printers, which are helpful to document findings for later review and to increase confidence of diagnosis.

The procedure involves palpating the mare for pregnancy at 60 to 70 days post-ovulation. The veterinarian will insert the transducer until a fetus is found. When the fetus is located, the entire fetus is scanned, visualizing the head and determining the dorsoventral orientation of the fetus. The fetal positioning can be detected by finding the direction of the mandible (it points ventrally and posteriorly) or finding the heart on the ventral midline.

The veterinarian will proceed posteriorly to the abdominal attachment of the umbilicus. Just posterior to the abdominal attachment of the umbilicus is the tubercle of the male fetus. To identify a female, the most posterior area of the fetus must be scanned. The female tubercle is best observed in a plane including the tailhead and the distal tibias or hocks. It appears just under the tailhead within a triangle formed by the tailhead and the two tibias. Definite identification of the female tubercle is difficult from any other plane.

Determinations are made in utero by locating the genital tubercle and defining its position relative to other fetal structures. The genital tubercle is the precursor to the penis in a male and the clitoris in a female. It is sonographically visible as a hyperechogenic, bilobulated structure about two millimeters in diameter that develops on the ventral midline between the hind legs. Around Day 55 of gestation, the tubercle migrates toward the umbilical cord in a male and toward the anus in a female.

Since we cannot determine the sex before approximately 60 days of gestation, sex determination is best diagnosed on a 60- to 75-day-old fetus.

After 75 days, the uterus is carried over the pelvic rim by the fluid of the pregnancy. The fetus moves to the most ventral part of the uterus and becomes difficult to reach by rectal ultrasound approach. The uterus, however, does not seem to move more ventrally into the abdomen. As the fetus develops, usually by 90 to 95 days, it seems to extend back into the pelvic cavity and again can be viewed sonographically. Frequently, fetal sex determinations can be done at 80 to 90 days gestation if the fetal position reveals the proper cross-sectional view.

At 90-140 days of gestation, the fetus has grown considerably and can be diagnosed about 75% of the time. However, at that time we are looking for external genitalia (glans penis, prepuce, mammary gland, teats, clitoris, etc.) These frequently are difficult to see because they are not well developed until around 110 days. Occasionally after 140 days the proper view can be attained and a determination can be made.

Accuracy of the sex diagnosis and the time required for the procedure is determined by the experience of the practitioner. Adequate time needs to be spent learning the ultrasonographic cross-sectional anatomy of a 60- to 140-day-old fetus. (Tip to veterinarians: The best way to learn cross-sectional anatomy is by doing 60-day pregnancy exams with the ultrasound, charging a normal palpation charge.)

Does It Work?

The results of my sex determinations performed in 1992 were 24 of 24 correct, with one mare in question. In 1993, 42 of 42 were correct, with four mares not reporting. In 1994, 70 of 70 were correct, with seven mares not reporting. In 1995, 178 of 178 were correct, with six not reporting. In 1996, 316 sex determinations were made. Time requirements made it difficult to check all 316 diagnoses, but there were no notifications of errors in gender diagnosis. The number of diagnoses in 1997 was 495. One was reported incorrect. (In that instance, a 65 day fetus was called a colt and the mare foaled a filly.) On two other occasions in 1998, I thought I saw a male tubercle until I checked the filly view for verification. In these two instances the tubercle of a filly was readily seen.

In 1998, approximately 620 sex determinations were done. Less than 2-3% of the time, determinations were not made. Usually because the mare was past 75 days, extremely fractious, or because the fetus was not 60 days.

In 1995, eight examinations were done with no diagnosis made. Of those eight, five mares were scanned more than 70 days after breeding. Two of the three mares were approximately 60 days from breeding, but their fetuses were much younger, approximately 52 or 53 days. Obviously, conception occurred seven or eight days post-breeding. They were re-examined a week later and a sex diagnosis was made.

Incorporation of sexual determination into breeding programs increased rapidly when owners realized that it was a very accurate and useful management tool. Much of the accuracy is due to the fact that there is no diagnosis made unless I am certain of fetal sex. I am certain when I find a definite tubercle in a filly or colt cross-sectional view, and reinforce it with a negative finding in the cross-sectional view of the opposite sex. When a tubercle is found, I move away from it and find it again several times.

Learning to determine fetal sex at 90 to 140-plus days requires another learning process because you are looking for different structures. External genitalia usually are more difficult to differentiate from surrounding tissues. Genitalia at 90 to 110 days are more difficult to differentiate because they are less developed.

In the female, the mammary gland is the most readily identifiable structure. It is found slightly more posterior to the abdominal attachment of the umbilicus than is the prepuce. The mammary gland begins to become visible around 90 days of gestation (usually), it shows on ultrasound as two hyperechogenic teats on a bilobed triangle of tissue that is slightly more dense than its surrounding tissue. If a mammary gland is found, I try to reinforce the diagnosis by finding teats or the clitoris. The clitoris will be a small hyperechoic structure on the posterior midline in the upper midtail area.

The prepuce in males, which is found in a very similar area, is a cone-shaped structure off the abdominal wall just posterior to the abdominal attachment of the umbilicus. The prepuce begins to be visible at about 85-90 days of gestation. The hyperechogenic point of the cone is caused by a prominent ventral median raphe of the prepuce. Frequently, the glans penis can be seen as a hyperechogenic area within the anterior part of the prepuce.

The difficulty with gender determination 90-140 days of gestation is primarily from the tissues being more difficult to differentiate. Large bones cast sonographic shadows over areas to be observed and the position of the fetus can make observation of the posterior area difficult. A diagnosis can be made about 80% of the time at this later stage and usually requires two to 10 minutes to perform. In contrast, at 60-75 days, a diagnosis can be made 99% of the time requiring 10 seconds to two minutes to perform.

After 140 days, the fetus is so large it is difficult to orient how the fetus is positioned because few landmarks will be visible in one field of view. Many sonographic artifacts can appear very similar to genitalia when seen alone (not in relation to other fetal structures).

With practice, a practitioner can be very accurate with sex diagnosis at 60-75 days of gestation, and from 90-140 days gestation. Accuracy should remain high from 60-140 days, but percentage of determinations of fetal sex made by the practitioner lessens from 80-140 days.

It is important to keep good records of the determinations of fetal sex to negate clerical errors. If a veterinarian cannot make a determination, he or she should not guess. Guessing leads to errors, and errors lead to loss of confidence in the procedure.

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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