Fetal Sex Determination

Figure 53-5 134-Day female mammary gland.

Figure 53-23 Bones of hindlimbs and pelvis.

CHAPTER 53Fetal Sex Determination

RICHARD D. HOLDER

Equine fetal sexing is the determination of sex of an equine fetus after 55 days of gestation. Different techniques for determination are required for the different stages of gestation. Technique 1, performed between 55 and 90 days of gestation, involves finding the genital tubercle (precursor to the penis in the male and precursor to the clitoris in the female) and determining its location relative to other fetal structures. The genital tubercle appears as a hyperechoic bilobulated “equals” sign 2 to 3 mm in length and develops 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 technique number one, the veterinarian should be able to make a diagnosis 95% of the time on the first examination with 99% accuracy. The examination should take from a few seconds to 5 minutes to perform.

Technique 2, performed between 90 days and 150 days of gestation, involves finding the external genitalia of the fetus (penis, glans penis, prepuce, and gonads in the male and mammary gland, teats, clitoris, and gonads in the female). At this stage (90 to 150 days) the veterinarian should be able to formulate a diagnosis 90% of the time on one examination with 99% accuracy. It should take from a few seconds to 10 minutes to perform the examination depending on the experience of the practitioner.

Around 150 days of gestation the fetus begins to assume an anterior presentation. This means that the head will be easily accessible, but the pelvic area will be out of reach. The fetus is much larger now and does not change positions often. Therefore rectal scanning for the determination of sex after 150 days is very difficult. The percentage of accurate diagnoses between 150 and 200 days would be very low (5% to 25% of attempts made by rectal examination).

Technique 3 involves performing a transabdominal scan and finding external genitalia after 150 days of gestation. In the author’s experience, determining sex transabdominally has a very low percentage of diagnosis and is very time consuming. In addition, clipping of the abdomen is sometimes required, and access to a more powerful ultrasound machine is helpful. Because of the low percentage of diagnosis, the extra amount of time required and clipping and ultrasound machine requirements, the author strongly recommends that techniques 1 and 2 be used for determination of sex in the equine.

The information obtained from the fetal sexing procedure is simply a management tool to be used by the owner or manager. Depending on the sire, the dam, or both, the fetal gender may affect the value of the foal. Consequently, this affects the value of the mare. (For example, 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.) This knowledge of the fetal gender could affect various management decisions such as where to foal the mare. (If you want to race a New York–bred colt, it is impractical to send a mare carrying a filly to New York to foal.) Some other considerations include cash flow predictions that could be determined earlier. Is the owner going to have a certain individual for sale (income) or an individual to race (expense)? Perhaps an owner wants a particular sex from a particular cross; if that breeding was successful, the owner could breed the following year to a different stallion, or the owner could try again to the same stallion if the desired sex did not result from the first mating. In this way the mating list could be determined much earlier. Perhaps if a mare is carrying a colt, the size of the fetus may be of some concern and precautions at foaling may be taken. Each year as the demand for fetal sex determination increases, there seem to be more new reasons for owners’ wanting to know the sex of the fetus.

If a practitioner is the only one in his or her area able to accurately perform a sexual determination, the procedure can develop into a new worthwhile profit center for the practice. Figure 53-1 is a graph of increasing demand when fetal sex determination was introduced into the area.

Figure 53-1 Graph of increasing demand when fetal sex determination was introduced into the area.

MATERIALS NEEDED FOR FETAL SEX DETERMINATION

1. Quality ultrasound machine with a 5-MHz linear array rectal transducer (ALOKA 500 SSD [Aloka Co., Ltd., Wallingford, Conn] or equivalent).

A. Settings: If the overall gain, near gain, or far gains are set too high, the contrast between the fetus and background is less and the tubercle is more difficult to see.

3. Ultrasound stand on wheels—for close eye-level viewing.

4. Subdued lighting—for good screen visualization.

5. Hat—to remove surrounding glare.

6. Fly spray (if needed) —to keep mare movement to a minimum.

7. Video or printers are helpful to verify the diagnosis and for record keeping.

FETAL DEVELOPMENT

See Tables 53-1 and 53-2 and Figure 53-2.

Table 53-1 Fetal Anatomical Development Chart

Days	Visualization
55-60	Fetus is very small; genital tubercle is difficult to see and may or may not be fully migrated.
60-70	Fetus is easily accessible; tubercle is distinct and fully migrated. (Easiest and most ideal time for fetal sex determination)
70-80	Fetal tubercle is distinct but slightly more difficult to reach.
80-90^*	Difficult to access fetus; tubercle is less distinct; genitalia development is just beginning. (Most difficult time) (Figure 10-2)
90-100	Fetus is generally accessible; genitalia are not very well developed. (Difficult to differentiate genitalia at times)
100-110	Genitalia becoming more evident.
110-120	Fetus is very accessible, and genitalia is well developed. (Ideal time in technique 2 for determination)
120-140	Genitalia are well developed, but posterior of fetus may be difficult to access at times.
140-150^†	At times fetus has anterior presentation with posterior out of reach.
150+	Usually the fetus has anterior presentation, and posterior of fetus is out of reach. Transrectally this stage is difficult, with a low percentage diagnosis of 5-25%.
150 to Term	Transabdominal—low percentage diagnosis, time consuming, need to clip abdomen, more powerful ultrasound machine is helpful.

* At 80 to 90 days the fetus is often difficult to reach due to the position of the uterus in the posterior abdomen. At approximately 80 days the allantoic fluid of the pregnancy pulls the uterus over the rim of the pelvis. The fetus is small, falls to th e most ventral portion of the uterus, and is very difficult to reach. As the uterine contents (fetus and fluids) increase in size, the uterus actually elevates more in the abdominal cavity and becomes easier to reach and view the fetus.

† Mares that are 130 to 150 days that are out of reach should be viewed again for possible position change.

Table 53-2 Ultrasonic Cross Sectional Anatomy Found in Planes I, II, and III

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Jun 4, 2016 | Posted by admin in EQUINE MEDICINE | Comments Off

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