Mother Nature clearly does not favor the birth of equine twins, as a significant number of twin embryos spontaneously abort within the first six weeks of pregnancy. Of the twin conceptions present after 40 days of pregnancy, about 80% will subsequently abort, most often after the eighth month of pregnancy, according to the University of California, Davis, Center for Equine Health. With late-term abortion, the mare can experience major complications including trauma, illness, infection, inflammation of the laminae (leading to founder), and reduced fertility for the next breeding. In the rare case where the mare delivers one or two live foals, there are increased foaling problems for the mare and greater loss of life for the foals during the first two weeks of life. The combined birth weight of the twins equals the size of one normal, single foal, and the twins never catch up to normal weight and size.
With the odds stacked so unevenly against the twin pregnancy, horse owners should endeavor to remove one of the twin embryos early in pregnancy. Doing so maximizes the chances of the mare continuing on with a single, healthy pregnancy.
Not Enough Room
Twin pregnancies in the mare nearly always occur when the mare ovulates an egg from each of two ovarian follicles and both eggs are fertilized, resulting in two embryos. The ovulations might occur at the same time, or a couple of days apart. In horses, it is rare that a fertilized egg splits to form identical twins.
Twin embryos either occupy their own uterine horn (paired branchings of the uterus leading from the body of the uterus to the oviducts) or share a horn. "If two embryos are lying next to each other in the same horn, one of the embryos has a 50-60% likelihood of reabsorbing or dissolving on its own by 40 days of pregnancy," says Michelle LeBlanc, DVM, Dipl. ACT. "The reason is that a specific part of the embryo must be in contact with the floor of the uterus. If this portion of the embryo does not have the specific contact, the embryo doesn’t receive enough nutrition and dies on its own. So, if one embryo sits on top of the other one, the one on top would be reabsorbed."
Spontaneous resorption is very rare when embryos occupy separate horns.
Embryos travel as part of their early life, and even if fertilized in the same horn, might separate on their own. Those that do, along with those that occupy separate horns, develop into twin fetuses that compete for nourishment.
Explains Mats H.T. Troedsson, DVM, Dipl. ACT, "The entire uterine surface is required in order to provide enough nutrition and oxygen to the growing fetus. If there are two instead of one, there will be less than half of the nutrition available for each fetus." As a result, abortion of both fetuses is likely to occur in late pregnancy, although the triggering mechanism isn’t exactly known.
"Different options are reported," Troedsson states. "One fetus occupying one horn may push the other fetus out of the other horn; the displaced fetus will outgrow its own placenta, starve to death, and trigger abortion of both. Another theory is that if the fetuses both occupy one horn each, there is less than half of the placenta for each one, and both are starved out."
In that tiny percentage where one or both twins live past delivery, the severely reduced nourishment throughout fetal development results in severely undersized animals.
"The combined birth weight of these foals is the same as the singleton. They do not catch up and always remain frail," Troedsson says.
One Is The Number
Reducing the twin pregnancy is the best option for preserving the pregnancy, and the optimal time for reduction is early in the pregnancy.
All bred 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, " Troedsson says. "If, for example, one ovulation occurred 11 days before examination, there may be another embryo present that is nine days old and not detectable by ultrasound. Therefore, most practitioners try to do twin ultrasound exams around 14 days after detected ovulation. This is the normal time to do the pregnancy evaluation because if the mare is not pregnant, she needs be rebred at about 19 or 20 days."
If the mare has a history of twinning, LeBlanc recommends a second ultrasound exam at 18 days post-breeding.
Recommended procedure for reducing a twin is to crush one of the embryos. "Before 25 days, if there is one in each horn, you can manually crush one with your hand," says LeBlanc, who added 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," she said. "We don’t know why, but one theory is that the fluid released from the crushed embryo gets underneath the re-maining embryo and lifts it off the floor of the uterus, causing it to lose its contact with the endome-trium 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 explains. "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 is released from the uterine lining and 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 com-pared 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 states. "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. That movement stops at Day 16."
If separation doesn’t occur by then, the veterinarian might inject the mare with prostaglandin to remove both pregnancies, bring the mare back into heat, and 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 secrete hormones that prevent her from coming back into heat for 120 days.
At that point, some veterinarians might try a manual reduction anyway. "If that fails and both embryos are crushed, the mare comes back into heat and is bred again," notes Troedsson.
Success rates for reducing one twin and saving the other diminishes past Day 25 whether they are separated or not, but attempts can be made. One method is to attach a needle to the end of an ultrasound probe, insert the needle into one of the vesicles, and either suck the fluid out of the vesicle or inject a toxic substance into the vesicle, causing the death of one twin. However, at that stage, the death of one fetus often triggers abortion of the other.
"This technique would be performed from 25 days up to 40 to 60 days, and is about 60-70% successful if there is one embryo or vesicle in each horn, depending on the operator," reports LeBlanc. "But where there are two in one horn, success is very, very low. Zero percent for most people, although Jonathan Pycock, BVM, PhD, DESM, MRCVS, a veterinarian in England, has a success rate of about 30%. This technique is primarily performed in mares where the pregnancy is very valuable."
Another method is to wait until about 110 to 120 days to utilize a transabdominal approach. Explains Troedsson, "I put a needle through the abdomen of the mare into the heart of the smaller fetus if there is a size difference, and inject 10 ccs of potassium chloride into the heart. There is about a 40-50% chance that the mare will abort one fetus and not both."
Surgical removal of one twin is possible, Troedsson says. "The success rates are very variable, but surgery is most successful when the fetuses are in different horns."
Twins occur a little more commonly in Thoroughbreds than in other breeds; about 20% of ovulations in Thoroughbred mares are double ovulations.
"Overall, the incidence of double ovulations is about 20%, half of which will result in a twin pregnancy by Day 14," reports Troedsson. "If we calculate spontaneous reduction, we’re left with an incidence of twin pregnancies between 2% and 5%. The majority of these cases end in abortion, with less than 1% giving birth to live foals."
Mares which produce twins once are likely to produce twins again.
Unfortunately, there is no good way to prevent or minimize twins in pregnancy.
"You could wait for one of the follicles to ovulate, then breed the mare," says LeBlanc. "However, she may not ovulate a second follicle on that particular cycle and therefore won’t get pregnant with that breeding."
Or, notes Troedsson, "The mare may ovulate both follicles at the same time, so you would lose the breeding."
Troedsson reports that some have tried reducing twins through reduced feed intake. "A study shows this increases the chance of spontaneous reduction of one fetus," he says. "However, the study hasn’t been repeated, and it needs to be confirmed. The question is, how much do you reduce the feed intake to reduce one–but not both–fetuses? I don’t think this is a very practical way to accomplish reduction."
Ultimately, the best option for a single pregnancy or, secondarily, an early reduction of both twins and a subsequently quick return to estrus, is to focus on early detection.
"Where we run into problems is when the mare is not checked before 16 days," emphasizes LeBlanc. "If the exam is not performed early, the success rate for reducing twins to a singleton become less and less.
"Do not skip that early ultrasound exam," she warns, "because that is the one that identifies if there is going to be a problem. A twin pregnancy doesn’t have to be a big problem."