After You Breed—Horse Breeding Articles
Fetal Sex Determination
by Richard D. Holder
Fetal sex determination is a new management tool being incorporated into many farm breeding programs. Depending on the sire and dam, the fetal gender may affect the value of the foal and the mare. If the value of the mare changes depending on the gender of the foal then appraisals, sales reserves, insurance coverage, and collateral limits for loans could all be affected. Knowledge of the fetal gender could impact management decisions such as where to foal the mare. If an owner wishes to race a colt bred in New York, it is impractical to send the mare carrying a filly to foal in New York. Other considerations include cash flow predictions in determining whether the offspring will be for sale (income) or to race (expense). An owner may want a particular sex from a particular cross. If the sex of the foal is what the owner desires, then it would be feasible the following year to breed the mare to another stallion. If the foal is not the desired sex, then the owner would again breed the mare to the same stallion. If a mare is carrying a colt, the size of the fetus may be of some concern, requiring extra precautions at foaling. Each year as the demand for fetal sex determination increases, there are more reasons for mare owners wanting to know the sex of the fetus.
Different techniques of ultrasound examination (scanning) are required for the varying stages of gestation. A technique using a rectal probe, performed at between 55 and 90 days of gestation, involves finding the genital tubercle, precursor to the penis in the male and the clitoris in the female, to determine its location relative to other fetal structures. The genital tubercle appears as a hyperechoic bilobulated equal sign 2 to 3 millimeters in length between the hind legs on the ventral midline of both sexes at 50 to 55 days. Migrations of the tubercle that indicate the sex of the fetus begin about day 54 to 55. A determination cannot be made before 55 days, because the migration toward the umbilical cord in the male or toward the anus in the female has not yet occurred. Using this technique, the veterinarian should be able to make a diagnosis 95% of the time on the first examination with 99% accuracy, which should take a few seconds to five minutes to perform.
A second technique, also using a rectal probe, is performed between 90 days and 150 days of gestation involves finding the external genitalia of the fetus, the penis, glans penis, prepuce, and testes in the male and mammary gland, teats, clitoris, and ovaries in the female. At this stage the veterinarian should be able to provide a diagnosis 90% of the time on one examination with 99% accuracy. It should take from a few seconds to 10 minutes to perform depending on the experience of the practitioner.
At around 150 days of gestation, the fetus begins to assume an anterior presentation, at which stage the head is easily accessible but the pelvic area is out of reach. The fetus is much larger and does not change positions so often. Therefore, rectal scanning for the determination of the sex of the fetus after 150 days is very difficult. The percent of diagnosis by this method between 150 to 200 days is low – 5% to 25% of attempts made.
The third technique involves performing a transabdominal scan and identifying the external genitalia after 150 days of gestation. In the author’s experience, determining the sex of the fetus transabdominally has a very low percentage of diagnosis and is time consuming. Additionally, clipping of the abdomen is sometimes required and requires access to a more powerful ultrasound machine. It is therefore recommended that techniques one and two be used for determination of fetal sex in the equine.
Learning these techniques provides a worthwhile service to clients but requires many hours of actual sonographic visualization of the equine fetus between 55 to 150 days of gestation. Only when the sonographic cross-sectional anatomy of the fetus is learned, will confidence of an accurate sexual determination be achieved.
From Equine Disease Quarterly
Contact Richard D. Holder, (859) 255-8741, rdhtex@worldnet.att.net
Hagyard-Davidson-McGee Assoc., Lexington, KY.
Different techniques of ultrasound examination (scanning) are required for the varying stages of gestation. A technique using a rectal probe, performed at between 55 and 90 days of gestation, involves finding the genital tubercle, precursor to the penis in the male and the clitoris in the female, to determine its location relative to other fetal structures. The genital tubercle appears as a hyperechoic bilobulated equal sign 2 to 3 millimeters in length between the hind legs on the ventral midline of both sexes at 50 to 55 days. Migrations of the tubercle that indicate the sex of the fetus begin about day 54 to 55. A determination cannot be made before 55 days, because the migration toward the umbilical cord in the male or toward the anus in the female has not yet occurred. Using this technique, the veterinarian should be able to make a diagnosis 95% of the time on the first examination with 99% accuracy, which should take a few seconds to five minutes to perform.
A second technique, also using a rectal probe, is performed between 90 days and 150 days of gestation involves finding the external genitalia of the fetus, the penis, glans penis, prepuce, and testes in the male and mammary gland, teats, clitoris, and ovaries in the female. At this stage the veterinarian should be able to provide a diagnosis 90% of the time on one examination with 99% accuracy. It should take from a few seconds to 10 minutes to perform depending on the experience of the practitioner.
At around 150 days of gestation, the fetus begins to assume an anterior presentation, at which stage the head is easily accessible but the pelvic area is out of reach. The fetus is much larger and does not change positions so often. Therefore, rectal scanning for the determination of the sex of the fetus after 150 days is very difficult. The percent of diagnosis by this method between 150 to 200 days is low – 5% to 25% of attempts made.
The third technique involves performing a transabdominal scan and identifying the external genitalia after 150 days of gestation. In the author’s experience, determining the sex of the fetus transabdominally has a very low percentage of diagnosis and is time consuming. Additionally, clipping of the abdomen is sometimes required and requires access to a more powerful ultrasound machine. It is therefore recommended that techniques one and two be used for determination of fetal sex in the equine.
Learning these techniques provides a worthwhile service to clients but requires many hours of actual sonographic visualization of the equine fetus between 55 to 150 days of gestation. Only when the sonographic cross-sectional anatomy of the fetus is learned, will confidence of an accurate sexual determination be achieved.
From Equine Disease Quarterly
Contact Richard D. Holder, (859) 255-8741, rdhtex@worldnet.att.net
Hagyard-Davidson-McGee Assoc., Lexington, KY.
