Fetal Development at Different Stages

1. 55–60 days.  The fetus is very accessible but it is small. The genital tubercle is difficult to see, and the tubercle may or may not have fully migrated.
   
2. 60–70 days.  This is the ideal time for the examination. The tubercle is distinct, fully migrated, and the fetus is easily accessible for viewing.
   
3. 70–80 days.  The fetus is slightly more difficult to reach and the tubercle is slightly less distinct.
   
4. 80–90 days.  It is more difficult to view the fetus, the tubercle is less distinct, the genitalia development is just beginning, and the fetus is frequently out of reach. Maiden mares often can be done at this time.
   
5. 90–110 days.  The fetus is usually accessible and the genitalia are gradually becoming more evident.
   
6. 110–120 days.  This is the ideal time for examination at the mid-gestation stage. The fetus is very accessible and the genitalia are very well developed.
   
7. 120–140 days.  The genitalia are well developed, but the posterior of the fetus may be difficult to access.
   
8. 140–150 days.  The fetus is large with the posterior difficult to access.
   
9. 150+ days.  The fetus is large with an anterior presentation and the posterior is usually out of reach transrectally. The transabdominal approach is best after 150 days of gestation.

Fetal Sexing 55–90 Days of Gestation

This stage involves finding the genital tubercle, a small, ∼2 mm, hyperechoic bilobed structure, resembling an equal sign. It is the precursor to the penis in the male and the clitoris in the female. The genital tubercle must be identified and located in relation to other fetal anatomic structures. The tubercle can be seen as early as 52 or 53 days of gestation located between the hind legs on the ventral midline. As the fetus ages the tubercle appears to migrate toward the umbilical cord in the male and toward the tail in the female. Around day 55 from conception the tubercle has usually migrated enough to make a determination. At this time the fetus is easily accessible by transrectal palpation and an accurate determination (99%+ accuracy) can be made a very high percentage (∼98%) of the time with one examination. This examination of the fetus takes 10 seconds to 2–3 minutes depending on the experience of the examiner. The fetus is easily accessible until around 75 days of gestation. At this time, in some mares, the fetus can be difficult to access. Usually, it can be done until about day 80.

At about 80 days of gestation, the fluid of the pregnancy pulls the uterus over the rim of the pelvis. The fetus is small and falls to the most ventral part of the uterus in the allantoic fluid. However, after ∼90 days, the uterus does not continue to move more ventrally into the abdomen. It actually elevates in the abdominal cavity as it becomes larger and the fetus is usually visible again at about 90–95 days of gestation. As the fetus grows it often extends back into the pelvic cavity and is readily visible after around 100 days. At this time the external genitalia are used to make a sex determination (Figures 18.1, 18.2).

Fetal Sexing 90–150 Days of Gestation

At 90 days the fetus comes back into view, as the uterus seems to elevate dorsally in the abdominal cavity and is easily accessible. The genital tubercle is less prominent now but the external genitalia (penis, prepuce, glans penis, and the gonads in the male, and the mammary gland, teats, clitoris, and the gonads in the female) are just beginning to be evident. Determinations are somewhat more subjective at this stage because of the need to evaluate the tissue densities between prepuce and muscle or mammary gland and muscle. Other fetal tissues frequently obscure the genitalia. The optimum time for determination at this stage is between 110 and 125 days. It is usually not as clear as the genital tubercle evaluations, and it may take anywhere from 30 seconds to 5–10 minutes. A sex determination can be made 90% of the time at this stage, with 98+% accuracy, with one examination.

Fetal Sexing after 150 Days of Gestation

If the posterior part of the fetus cannot be accessed transrectally, it is worthwhile to attempt transab­dominal ultrasonography using a 2.5–3.5 MHz sector scanner. With the proper equipment good visualization of the posterior area of the fetus can frequently be achieved. The transducer should be placed on the midline of the mare’s abdomen slightly anterior to the udder and moved anteriorly until the fetus is located. Locate the heart of the fetus for orientation and locate the umbilical entrance to the abdomen. Follow the ventral midline of the fetus posteriorly from the umbilical entrance to the abdomen until the base of the tail is reached. In the male, the prepuce and hyperechoic area of the glans penis will be seen just posterior to the entrance of the umbilical cord. The mammary gland and teats of a female fetus will also be seen in this area. Moving posteriorly it is possible to see a clitoris on the fetal midline before the base of the tail is reached.

The external genitalia on a transabdominal scan appear the same as the 100+ day transrectal views but they are not quite as clear. When attempting transabdominal fetal sexing, the heart and the umbilical entrance into the abdomen are the most useful markers. Adjusting the ultrasound to a deep penetration reduces the visual size of the fetus and allows for better visualization of a larger area of the fetus. This is helpful in relating the location of certain structures to other fetal anatomic structures. The genitalia can be seen on the ventral midline, between the umbilical entrance to the abdomen and the base of the tail. These include the prepuce, glans penis, and penis in a colt and the mammary gland, teats, and clitoris in a filly. These structures are difficult to visualize but can be seen with much practice.