Fetal Sexing: Is it a Colt or Filly?

There are many reasons to want to know the sex of the fetus your mare is carrying, especially if she is headed to the sales. In this article from Rood and Riddle Equine Hospital near Lexington, Kentucky, you will learn the basics of fetal sexing.

There are many reasons to want to know the sex of the fetus your mare is carrying, especially if she is a registered mare headed to the sales. In this article from Rood and Riddle Equine Hospital near Lexington, Kentucky, you will learn the basics of fetal sexing from founding partner Tom Riddle, DVM.

The determination of the sex of a mare’s fetus, or fetal sexing as it is commonly called, is an extremely accurate procedure performed by rectal ultrasound examination of the pregnancy. With an experienced practitioner, the correct determination is made in well over 99% of cases.

The procedure is difficult to perform, not because it is challenging to interpret the ultrasound image, but rather because it is difficult to place the ultrasound probe in the position necessary to view the required image. Most mares tolerate the procedure very well, but in some cases it does require a slightly deeper placement of the ultrasound probe than in the routine pregnancy ultrasound.

In my practice there are two windows of opportunity for fetal sexing, the first between 58 and 76 days of gestation, and the second between 110 and 130 days. In some occasions the sex may bthat which is too young, the anatomical structures needed for the determination may not be mature enough. If one attempts the procedure too late, the fetus may be dropped down too far in the abdomen to reach the needed structures.

I prefer the earlier window (58-76 days) because the fetus is almost always in a location that permits the necessary view of the anatomy. With the second window, approximately 15% of the time the fetus cannot be viewed properly and a repeat examination at a later date is needed. Because a second examination may be needed, it is best to check mares early in the second window to allow time for a second examination.

In the first time period (58-76 days) the sex is determined by identifying the location of a structure called the genital tubercle, which is present in both colts and fillies. In colts the tubercle will develop into the penis, and in the filly the tubercle becomes the clitoris. In the second window (110–130 days) the anatomical structures have matured and the determination is made by identifying the penis in the colt and the udder in the filly. Fetal sexing

is performed for a variety of reasons, ranging from appraising the value of the pregnant mare to just plain curiosity. A partial list of reasons for fetal sexingincludes:

  • Valuation of mare prior to selling;
  • A factor in deciding who the mare should be bred to the following year (some clients want a foal of a particular sex by a certain stallion);
  • A client may ship mares carrying only colts (or fillies) to foal in another state or country; for example, the Japanese frequently will ship only mares carrying colts to Japan;
  • Agents may have an order for mares carrying only colts or only fillies (many European agents look for well-bred mares carrying fillies)
  • A factor in deciding whether a mare will be sold.

Probably the most common reason for requesting fetal sexing is to provide prospective buyers at the broodmare sales with the sex of the mare’s fetus. Whether the mare is carrying a colt or a filly may have a significant effect on her value. Some buyers may prefer a filly by a sire known for siring good female runners or a filly from an impressive female family, while many buyers prefer a colt because colts on average bring more money when selling as weanlings or yearlings. An analysis of the past 10 Keeneland September Yearling Sales shows colts averaging 25.7% more than fillies (thanks to Mark Taylor of Taylor Made Farm for providing this information).

Providing prospective buyers with the sex of the foal the mare is carrying gives the buyer one more piece of information to factor into the price he/she is willing to pay for the mare.

In these challenging economic times, spending money wisely is more important than ever. The decision to fetal sex a mare should be made based on the mare owner’s circumstances and the intended use of the information. In many cases the relatively small investment in the cost of fetal sexing ($135 in our practice) can generate a significant return for the owner.






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