Tom Riddle, the son of a veterinarian, was born and raised in Greenville, S.C. He graduated from the University of Georgia College of Veterinary Medicine in 1978 and began working at Spendthrift Farm. In 1982, Riddle joined Bill Rood in his ambulatory practice in Lexington which was the beginning of Rood & Riddle. Dr. Riddle has served as president of the Kentucky Association of Equine Practitioners, the Society for Theriogenology and the Theriogenology Foundation. He was the first person to recognize and report the 2001 abortion outbreak which became known as Mare Reproduction Loss Syndrome. Clinically, he has special interests in uterine culture and cytology, early twin management, fetal sexing, and diagnostic ultrasound of the mare’s reproductive tract. Riddle currently has a busy breeding farm practice in Central Kentucky.
Q - How does knowledge of a fetus’ sex help the horse owner or breeder? What are possible benefits? Example?
A - By far the majority of the mares that I fetal sex are Thoroughbreds, therefore, my remarks will be based on my experience in that industry. Breeders want to know the sex of the fetus their mare is carrying for a variety of reasons, including selection of a stallion for the following year (e.g. They want a colt by a certain stallion and the mare is carrying a filly), making financial plans (e.g. Some breeders sell all colts and keep fillies), and sometimes they just want to know. By far though, the most common reason for fetal sexing is as an aid in placing a value on the mare.In many cases the knowledge of the sex of the mare’s fetus is used to help determine the value of the pregnant mare. For example, if the owner is trying to make a decision on selling the mare, he/she knows that at the yearling sales, colts usually sell for about 20% more than fillies, which means that a mare carrying a colt may be more valuable (there are also situations when a filly has more value, such as when the mare is from a strong female family and/or the covering stallion is a known sire of good race fillies or good broodmares.) There are many other factors that affect the sale price of a mare, including pedigree, conformation, race record, age, reproduction history, and covering stallion; however, in an industry where large sums of money change hands, many smart buyers and sellers want to have as much information as possible on which to base their transaction decisions; therefore, the sex of the future foal of a mare may be very important in deciding on her appraised value.
Q - Is it correct that at the stage of gestation when fetal sexing is done, if the pregnancy is terminated, the mare cannot be brought back into cycle and bred that year in time for a normal foaling date the following year?
A - Yes, the earliest time in gestation that fetal sexing can be performed is approximately 56 days, which means that if the pregnancy were terminated after fetal sexing, it would be extremely unlikely that the mare could be bred back successfully during the regularly scheduled breeding season. I am thankful that I have never been asked to end a pregnancy because the owner was not pleased with the sex of the fetus the mare was carrying. The owners that I deal with have always felt that a foal, colt or filly, was far more valuable than no foal at all. Mares that lose their foal for any reason after 36 – 40 days will generally not cycle back to be rebred in a timely fashion because structures called endometrial cups have formed in the mare’s uterus, and normal ovulations will not occur for approximately the next 60 – 90 days.
Q - How is it done? What kind of equipment is used for Fetal Sexing procedure? Are there any safety issues which need to kept in-mind before attempting Fetal Sexing?
A - In my practice I perform fetal sexing using a digital ultrasound (Sonosite Turbomax) per rectum. Any good quality rectal ultrasound probe is adequate and for many years I used a linear 5 mhz probe, although the clarity of the digital ultrasound image is much better. For a veterinarian experienced in fetal sexing the procedure is relatively fast (one – three minutes) once the rectum is evacuated of feces. Once the fetus is identified at 56 – 74 days of gestation, the veterinarian finds the genital tubercle, the “precursor” of the penis or clitoris, and determines the location of the tubercle relative to other anatomical structures, allowing him/her to diagnose colt or filly. As with any rectal examination of an equine, care must be taken to avoid perforation of the rectum. Otherwise, I feel that the procedure is very safe for both the mare and the fetus. Ideally fetal sexing should be done in a mare that is properly restrained for a rectal palpation, in an area with minimal distractions and out of direct sunlight.
Q - At what stage in the pregnancy should the procedure be performed?
A - The first window for fetal sexing is from 56 days to 74 days. I feel that this is definitely the best time for this procedure because the fetus can almost always (99+% of the time) be viewed in the proper plane to make a diagnosis. After approximately 75 days and up until about 110 days, the uterus and fetus descend ventrally (drop down) and the fetus is usually not visible on ultrasound. By 100 – 110 days the fetus becomes larger and can be seen on rectal ultrasound. Fetal sexing can then be done in later gestation (>110 days), but approximately 15% of the time the proper plane for diagnosis cannot be seen, which means the sex cannot be determined. In this second window, the veterinarian is looking for the penis in the male and the udder or clitoris in the female. In some cases the testicles or ovaries may be seen. In some cases in later gestation fetal sexing can be done transabdominally (the ultrasound probe is placed on the skin of the mare’s abdomen instead of in the rectum), but again the proper view for diagnosis of the sex cannot always be found.
Q - Does It Work?
A - With an experienced operator fetal sexing is very accurate. While I would never state that any procedure is infallible, I do feel that the correct diagnosis of the sex of the fetus is made greater than 99% of the time. I have done thousands of fetal sexings in the first window and have had only two that were reported to have been wrong.
Rood & Riddle is a worldwide leader in equine healthcare offering innovative and highly skilled treatment of horses. Established in Lexington in 1982 as an ambulatory practice, Rood & Riddle became a full service equine hospital in 1986, serving as a referral center for horses requiring specialized medical and surgical care and providing ambulatory care for all breeds and disciplines. Rood & Riddle has recently expanded its Lexington, Ky location with the addition of a 208 acre reproduction facility. This addition, as well as satellite hospitals in Saratoga, NY and Wellington, FL, allows Rood & Riddle to uphold an unwavering commitment to quality, both in the care of horses and in relationships with clients and the community.