Determination of Fetal Gender

By | 2012-10-24

Fetal gender determination has been incorporated into the management programs of many breeding farms. Depending on the sire or the dam, the fetal gender may affect the value of the fetus and therefore influence the value of the pregnant mare. This knowledge could change various management decisions such as appraisals, foaling location, sales’ reserves, insurance coverage, collateral limits for loans, mating lists for the next year, and buy/sell decisions.

Table Development of Ultrasound Findings that Indicate Fetal Gender shows what to expect at the different stages of fetal development. The basis for sex determination when the fetus is between 55 and 90 days’ gestation is the location of the genital tubercle — a bilobulated hyperechoic structure 2 to 3 mm in length. This structure resembles a brightly colored “equals” symbol (), and is the precursor for the penis in the male and the clitoris in the female. The tubercle develops between the hind legs on the ventral midline in both sexes and at approximately day 53 or 54 of gestation appears to begin migrating toward the umbilical cord in the male and toward the anus in the female. Location of the tubercle between 55 and 90 days’ gestation enables the practitioner to determine the gender.

Table Development of Ultrasound Findings that Indicate Fetal Gender

Day(s) Ultrasound Findings
55-60 Fetus is very small; genital tubercle is difficult to see; tubercle may or may not be fully migrated.
60-70 Ideal time for examination — fetal tubercle is distinct and fully migrated; fetus is accessible for viewing.
70-80 Fetus becomes more difficult to reach.
80-90 Most difficult time to view fetus — tubercle is less distinct; genitalia development is just beginning; fetus is frequently out of reach.
90-100 Fetus is generally accessible, but genitalia are not very well-developed.
100-110 Genitalia are becoming more evident.
110-120 Ideal time — genitalia is well-developed.
120-140 Genitalia is well-developed, but posterior of fetus may be difficult to access at times.
140-150* At times the fetus has anterior presentation with posterior out of the examiner’s reach.
150+ Usually the fetus has anterior presentation, and the posterior of the fetus is out of the examiner’s reach.

*Mares of 130 to 150 days’ gestation that are classified as being “out of reach” should be viewed again for possible position changes.

When the fetus has reached 55 to 80 days’ gestation, a veterinarian should be able to make a gender diagnosis 95% of the time with one examination. The accuracy of fetal sexing should reach 99%, and the time required to make the determination should range from a few seconds to 5 minutes depending on the experience of the clinician. At 80 to 90 days the fetus is temporarily difficult to reach due to the positioning of the uterus in the posterior abdomen. At approximately 80 days the fluid of the pregnancy pulls the uterus over the rim of the pelvis. The fetus is small, falls to the ventral portion of the uterus, and is difficult to reach. As the fetus grows, the uterus actually elevates more in the abdominal cavity and the fetus becomes easier to reach and view (). After 90 days’ gestation, the tubercle becomes less distinct and more difficult to see. Therefore, the clinician must rely on developing external genitalia — in the female, the mammary gland, teats, and clitoris (), and in the male, the penis and prepuce () — to make the gender diagnosis. Consistent differentiation between male and female gonads at differing stages of gestation is difficult (). Consequently, gonads are used only for the reinforcement of a diagnosis, not for the diagnosis itself. At 90 to 150 days of gestation, a veterinarian should be able to formulate a highly accurate gender diagnosis 85% to 90% of the time. The diagnosis should require a few seconds to 10 minutes to perform, again, depending on the experience of the practitioner.

An attempt to sex the fetus should not last for more than 10 minutes per session on any one mare, because depending on the type of restraint used she may become fractious. No person or mare should experience injury during this elective procedure. If the mare becomes fractious, the veterinarian should stop and attempt the procedure on another day.


A high-quality ultrasound machine with a 5-MHz linear array rectal transducer (ALOKA 500 SSD [Aloka Co, Ltd., Wallingford, Conn.] or equivalent) is essential. If the overall gain, near gain, or far gain are set too high, the contrast between the fetus and background is less and the tubercle is more difficult to see. For optimal close eye level viewing, an ultrasound stand on wheels, subdued lighting for good screen visibility, and a hat are recommended. Video recordings or printers are helpful to verify the diagnosis and for record keeping.

Rectal palpation essentials include lubricant and sleeves and adequate restraint that might include a twitch, stocks, or tranquilizer. Depending on the situation, use of tranquilizers is acceptable, but this may cause the uterus to relax and drop away from the examiner and become more difficult to reach. This author uses xylazine (200 mg IV) mixed with butorphanol tartrate (10 mg). Propantheline bromide (30 mg IV) may be used to prevent rectal straining. Fly spray (if needed) will help to keep mare movement to a minimum.


The procedure involves a thorough evacuation of the feces from the rectum to allow easy manipulation of the transducer. The clinician can determine the position of the fetus by scanning the entire fetus. The skull is a good anterior marker, the heart a good ventral marker, ribs coming off the vertebrae and the base of the tail are good dorsal markers, and the tail is a good posterior marker.

Once the position of the fetus has been determined, the veterinarian should proceed with the transducer to the posterior part of the fetus until the image of the latter has gone completely off the transducer. The veterinarian should gradually ease back onto the fetus with the transducer and cut a plane perpendicular to the axis of the spine of the fetus. A cross-section of the tailhead should be picked up on the dorsal aspect of the fetus. This cross-section will appear as a hyperechoic round mass with very little muscle tissue around it. On the ventral aspect of the fetus, two tibias, which appear as hyperechoic round structures with no muscle mass, should be seen to form a triangle with the tailhead. If the fetus is female, a hyperechoic tubercle will appear within the tibia-tailhead triangle with the tubercle slightly toward the tailhead. The female tubercle is difficult to consistently identify other than within the tibia-tailhead triangle If the fetus is a male, nothing will be seen within the tibia-tailhead triangle. If this is the case, the veterinarian should move the transducer gradually along the anterior aspect of the fetus, keeping the same perpendicular plane (plane II) to the axis of the spine of the fetus. A hyperechoic structure resembling an “equals” symbol should be seen between the two tibias, which when the transducer is moved anteriorly become the stifles or femurs (). It is important to remember that tibias have no muscle mass around them but femurs do have muscle mass around them. When the transducer is moved further anteriorly, the large round abdomen will be seen. The male genital tubercle can often be seen on the outside ventral wall of the abdomen just posterior to the urachus, which is seen as a dark hole 4 to 5 mm in diameter. When the transducer is moved back and forth over the posterior area in this plane a tubercle is usually seen.

The male tubercle can also be readily seen from a frontal plane, This plane exhibits the front legs, ventral abdomen, and hind legs with the tubercle appearing slightly anterior to a line drawn between the hind legs (usually femurs or stifles).

After 90 days of gestation, the tubercle is less distinct. The veterinarian should proceed to the posterior aspect of the fetus and find the point at which the posterior muscles of the buttocks come together and form a definite cleavage on the ventral midline. Next, the cleavage should be followed posteriorly to the tailhead. If the fetus is a female, a clitoris will appear as a small round structure in the cleavage shortly before the tailhead is reached. If the small round structure is too close to the tailhead, it could be the anus. If the midline or cleavage line is followed anteriorly, the mammary gland in the female or the prepuce and penis in the male are encountered. A mammary gland appears as a triangular, slightly denser tissue than surrounding muscle tissue. Each half of the gland may have two bright hyperechogenic teats and slightly dense areas; a translucent division between halves of the mammary gland may also be present. If the prepuce is viewed from a cross-section across the posterior ventral abdomen, it will appear as a cone-shaped structure off the ventral abdominal wall just posterior to the urachus. The prepuce may appear as a cone shaped structure with a hyperechoic area within the cone. The hyperechoic area is the penis itself. Sometimes the shaft of the penis can be seen with a hyperechoic distal segment .

When the fetus gets older, the genitalia become more developed and more easily differentiated from surrounding tissue. When the fetus develops, however, it becomes more difficult to access the posterior area. After day 150 of gestation, the fetus begins to have an anterior presentation that puts the posterior area out of reach. Also, because of its size the fetus is less apt to rotate the posterior part to a more accessible position. A fetal sex determination has been made at 184 days on trans-rectal exam, but this is very unusual and possibly not a good sign if the fetus is in a posterior presentation this late in gestation.

This procedure is for gender identification only and not for gender control. It would be unusual to have a mare successfully pregnant at 60 days, terminate the pregnancy, and be successfully re-mated that season.

Mastery of these sex determination techniques by veterinarians provides a worthwhile service to clients but requires many hours of actual sonographic visualization of the equine fetus at between 55 and 150 days of gestation. Only when the sonographic cross-sectional anatomy of the fetus is learned will a consistent, accurate diagnosis of the fetus’ sex be made.