Twins occur a little more commonly in the Thoroughbred than in other breeds, writes Marcia King in the March edition of The Horse
. Approximately 20% of ovulations in Thoroughbred mares are double ovulations. The best option for preserving the pregnancy is to eliminate one of the twins. The optimal time for reduction is early in the pregnancy.
All mares should have their first ultrasound exam for pregnancy performed between 14 and 16 days post-breeding.
"We can detect an embryonic vesicle (the fertilized egg with its surrounding fluid and membranes) as early as 10 to 11 days after the mare has ovulated," says Dr. Mats H.T. Troedsson of the University of Minnesota. "If, for example, one ovulation occurred 11 days before examination, there may be another embryo that is present that is nine days old and is not detectable by ultrasound. Therefore, most practitioners try to do twin ultrasound exams around 14 days after detected ovulation."
If the mare has a history of twinning, Dr. Michelle LeBlanc of the University of Florida recommends a second ultrasound exam at 18 days.
When there are two embryos, veterinarians usually crush one.
"Before 25 days, if there is one in each uterine horn, you can crush one with your hand," says LeBlanc, adding that the success rate at this stage is more than 90%. "After 25 days, it's more difficult to crush an embryo because they are larger, and there's a higher likelihood that if you crush one, the other will also die. We don't know why, but one theory is that the fluid released from the crushed embryo gets underneath the remaining embryo and lifts it off the floor of the uterus, causing it to lose its contact with the endometrium and its source of nutrition."
Crushing is a very quick procedure done without sedation if the mare is calm and has a pliable rectum.
"The veterinarian puts his or her hand through the rectum, reaches over, cradles the uterus in the hand, locates the vesicle, and crushes the embryo," LeBlanc says. "When it's crushed, it feels like a popped blister."
Some practitioners might give anti-
inflammatory drugs for this procedure to prevent the release of prostaglandin, the hormone that brings the mare into heat.
"If you manipulate the uterus too much, theoretically prostaglandin will be released," Troedsson says. "However, studies found that the use of anti-inflammatory drugs compared to using nothing both yielded excellent results, so in normal cases we don't give additional drugs."
When two embryos share a horn, crushing is not a good option.
"If you crush one, you're more than likely to crush the other," LeBlanc says. "However, the embryos travel from one horn to the other and are continually moving up to the age of 16 days, so the veterinarian can re-evaluate the mare in a half hour or so to see if the embryos have separated, then crush one of the two."
If separation doesn't occur by day 16, the veterinarian might inject the mare with prostaglandin to remove both embryos and bring the mare back into heat, then try to breed her again. This procedure must be performed by day 37 of the pregnancy because, at that time, the mare makes endometrial cups that prevent her from coming back into heat for 120 days.