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Fetal Sexing for the Practitioner
R.M. Tuner
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Introduction
Determination of fetal gender is possible between about days 55 and 70 and days 110 – 150 of gestation using transrectal ultrasound examination.1 Prior to day 55, the genital tubercle (the future penis in the male and clitoris in the female) is located in a relatively neutral position between the hind legs of the fetus. However, after about day 55, the tubercle migrates and becomes located closer to the umbilical cord in the male fetus (Fig. 1) and underneath tail head in the female fetus (Fig. 2). Identification of the genital tubercle and its position relative to the umbilical cord and tail head thus allows for identification of fetal gender. Beyond approximately days 80 - 90, the conceptus and the uterus descend over the pelvic brim into the caudal abdomen, usually placing the hindquarters of the fetus out of reach via the transrectal approach. However, around days 110 - 120, the fetus becomes more buoyant and once again can be examined successfully transrectally until approximately day 150. At this stage, gender determination is somewhat trickier and depends on identification of the external genitalia (penis, scrotum in the male; mammary gland, clitoris, vulva in the female). Nonetheless, with an experienced examiner, gender can be accurately determined over 90% of the time.1 Beyond approximately day 150 and until term, you also can perform fetal sexing via a transabdominal ultrasonographic approach; however, this is much more time consuming, requires specialized ultrasonographic equipment and generally is not as successful as the transrectal approach. This presentation will focus only on the transrectal approach that can be utilized between days 55 and 70 of gestation. This technique is becoming increasingly popular in practice to determine the sale value of a pregnant mare and/or to direct future breeding plans.
Restraint
The mare is restrained as for standard examination of the reproductive tract. Stocks are preferable and a dim environment is ideal to allow for better visualization of the ultrasound screen. It can be very helpful to sedate the mare for this procedure. Not only will it decrease straining and movement on the part of the mare, but it also will decrease movement of the fetus. A mixture of xylazine and acepromazine or detomidine and acepromazine usually works very well. When first learning the technique, the addition of a tocolytic such as buscopan or propantheline bromide also can be very helpful.
Equipment
The better your equipment, the easier this procedure will be. A high quality ultrasound machine with excellent resolution will greatly facilitate identification of the relatively small tubercle. A 5 or 7.5 MHz linear array transducer is ideal. Empty the rectum of manure and scan the uterus to first confirm that the mare is pregnant and then to identify the exact location of the fetus. Once you have identified the fetus, you must orient yourself as to which end of the fetus is cranial and which is caudal. You will obviously be focusing on the caudal end.
Technique
The goal is to obtain both frontal and transverse views of the caudal end of the fetus. The frontal view allows imaging of the genital tubercle in the male, and the transverse view can be used to see both the male and female genital tubercles (Figs. 3 and 4). For the frontal view, maneuver the transducer until you see long axis sections of the fetus. Usually you can obtain the desired plane with the probe on the dorsal surface of the uterus. But occasionally, depending on the location of the fetus, the transducer must be moved to the cranial or lateral face of the uterus to get the image just right.
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