Congratulations, you have successfully bred your mare. But do you have time to sit back, catch your breath, and wait eleven months for the foal? Unfortunately not. You might have cleared the first hurdle, but the obstacle course has just begun. The first two or three weeks after conception is the time to check your mare to see if she is carrying twins. It's important that you check for twins early in the pregnancy because chances are that if twinning has occurred, the outcome will not be in your favor.

The most common cause of twinning is from double ovulation.

"It's important to know whether a mare has double ovulated because it's a signal to look at her closely for twins," according to Walter Zent, DVM, who does reproductive work for the Hagyard-Davidson-McGee veterinary firm in Lexington.

With today's technology, you can find out if your mare is carrying twins within a few days of breeding--most can be detected as early as Day 11 or 12. Twins are easily detected until around Day 60, when they develop to a size that the pelvic rim interferes with visualizing the fetuses using rectal ultrasound examination, said Zent. Most veterinarians today have access to ultrasound equipment to scan mares for twins. Although ultrasound has been around for quite some time, it's only been in the last seven or eight years that it has become routine and affordable for a practitioner to own some of the smaller, portable machines.

With ultrasound, veterinarians will detect nearly 100% of all twins. Before ultrasound, veterinarians performed rectal palpations on mares from 28-30 days onward with decent results. But with the detection rate of ultrasound, it's better to use it in addition to rectal palpations.

"With rectal palpation alone, four or five sets of undiagnosed twins a year was common," said Zent. "That's because 80% of twins in the same uterine horn will be missed by rectal palpation if they are right next to each other. There would have been a lot more than that, but the death rate is much higher in those twins (in the same horn) than when they are in separate horns, so you are not going to see them because they are going to disappear." This is called self reduction, and it happens 80% of the time with pregnancies in the same horn, according to Zent.

"I like to scan my mares between 14-16 days (post-ovulation), then again at 28 days. In doing this, you are going to pick up 99.5% of all your twins," said Zent. "It's also important to know if she double ovulated, and if so, if it was synchronous or asynchronous. That's because if you scan the mare at 16 days and you see a nice 16-day pregnancy, you have got to remember this other embryo is only going to be 11 or 12 days old if the double ovulation was asynchronous, and it's going to be really small."

It's imperative to detect the twins as early as possible because they can delay a breeding program severely if not dealt with at an early stage. If the twins are detected from Days 14-16, then one of them can be manually reduced. By manually reducing one of the twins at this early date, it gives the other embryo a chance to fully recover and be carried to term normally. The reduced twin will be absorbed naturally if eliminated early enough.

Manually reducing, or crushing, one of the twins simply means destroying it. This can be accomplished in several ways. Zent, a proponent of action, not waiting, will eliminate the smaller twin early in the pregnancy. He accomplishes this by crushing the vesicle of the smaller twin.

"You rectally go in and pick up the mare's uterus, find the embryo, and pinch it until the vesicle breaks," said Zent.

Reducing needs to be done prior to Day 17, according to Tom Juergens, DVM, with the Anoka Equine Clinic, Elkriver, Minn., because, "After Day 17, it's a little more difficult because you can't move the twin within the uterus very easily without causing some disruption of the environment of the uterus. If you reduce one after Day 17 of pregnancy, there is a chance you may interfere with the other remaining embryo."

By waiting this long, your risk of losing both conceptions is greater. Juergens said some veterinarians have used the ultrasound probe to guide a needle placed through the uterine wall used to puncture one of the vesicles, eliminating it that way. Waiting until after Day 30 to manually reduce one of the conceptions is not really an option anymore, according to Juergens.

"A lot of mares will reduce one of the twins themselves by Day 30, so if you go back with the ultrasound machine, as many as half of all the twins detected at Days 14-16 will have been eliminated by the mare," said Juergens.

If twins are detected after Day 30, the best recourse is to terminate the pregnancy because after Day 35, it's hard on the mare's uterus. Catching the twins before Day 38 still gives you a good chance of getting the mare cycling and re-bred that same season. If you wait until after Day 38 to abort, you will have a hard time getting the mare back into heat, not to mention the extra stress on her uterus caused by aborting larger embryos. At this point, you have probably lost the whole year, and will have to wait until the next year to breed the mare.

Most mares will abort twins naturally by Day 200 because a mare's uterus is not designed to carry two pregnancies. If both survive, one or both of the twins will be developmentally compromised because they did not receive adequate amounts of oxygen and nutrients.

"One fetus always seems to get a larger share than the other one so there is usually a disparity in size," said Juergens.

If the twins are carried to term, which seldom happens, then you end up with 60-pound weaklings which will be compromised as adult athletes.

"Getting useful individuals out of twins is not common," according to Zent. "They will always be small. That's not to say there haven't been exceptions. There have been twins that have been good broodmares, but I don't know about good stallions. I certainly can't think of any that were good stallions. It would be a rare exception for them to be a successful racehorse."

The odds are even against delivering a live foal when dealing with twins that make it close to term--often one or both will be born dead. If they are both born alive, one often will die within a few days.

"If they get this far (close to term), what usually happens is one of them will starve to death and die," said Zent. "Then, depending on when it dies and where it is, sometimes the other one will go on and develop normally and be alright. However, if it happens too late the other one, if it is born alive, will still be too small because the placenta can't get over in the part of the uterus where the dead fetus is, or the mare will abort the dead one and it will take the live one with it. This is probably the most common thing that happens, and they both end up dying."

When the twins arrive, if they are both alive, some people will put the larger one on a nursemare. While Zent said this is not usually necessary, it does ensure that both twins are having their nutritional needs met. Zent said that mares which foal live twins tend to be big mares, anyway, and should have a sufficient supply of milk for both foals. The twins also will be smaller than single foals, and they will not require as much milk. Zent said some owners will use a nursemare to speed along the foals progress.

While there is no way to really prevent twinning from occurring, you need to be aware of the potential problem, especially if your mare has double ovulated, or twinned, in the past.

"There are female lines that double ovulate all the time," said Zent. "If a mare double ovulates, chances are that her daughters will double ovulate as well."

Some breeds also tend to be more prone to twinning. Zent, who works with Thoroughbreds and Standardbreds, said he sees a lot more twins in Thoroughbreds than Standardbreds.

Because there is no rule book when it comes to twinning, you should be prepared for whatever comes your way. You might end up with two small, but healthy, foals. However, chances are you will end up with a mare which has aborted the pregnancies. Abortion might be prevented if your mare is scanned early to detect the presence of twins. Remember, if twins are found with the initial scan at Days 14 to 16, one of them can be manually reduced and the remaining one should continue to develop normally.

Even if you detect twins around Day 30 and it's too late to manually reduce one of them successfully, you can still abort the pregnancy and re-breed your mare the same year. Just don't wait beyond Day 38, or you will have a tough time getting your mare back into heat that season, and the abortion at that point might do damage to her uterus since the embryos will be larger. If you wait that long, it probably will be another year before she conceives again.

If your mare delivers twins at or near term, Zent said, "It takes a uterus more time to recover from twins than it does for a mare that had a single foal. So they are not as fertile early (after foaling) as one that just had a single birth."

About the Author

Tim Brockhoff

Tim Brockhoff was Staff Writer of The Horse:Your Guide to Equine Health Care from 1995 to 1999. His degree is in Agricultural Communications from the University of Kentucky, and his equine experience is with American Saddlebreds.

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