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The Trouble with Twins

By Kenton H. Arnold, DVM
The Trouble with Twins

As we breed our mares and make plans for future foals, our utmost concern is to raise a healthy foal while keeping our mares healthy and safe.

Producing equine twins may be the furtherest thought from our minds, but in actuality, twinning happens to be the number one case of abortion in mares. Most twins are aborted before being carried to full term; however, those that do survive the entire pregnancy will have difficulty surviving the birth. Many are small and weak and may not survive to become productive athletes.

The probability of difficulty in giving birth, retained placenta and other complications greatly increases the risk that the mare herself may not even survive the delivery process. If she survives, getting this type of mare back in foal is sometimes difficult.

Occasionally one or both of the twins will survive. However, it is extremely prudent to do what it takes to insure that a set of twins never happens in your barn. Early detection is critical in preventing a twining catastrophe.

Most equine twins occur due to double ovulation. This can occur when two developing follicles release an ovum (egg) simultaneously. These developing follicles can be on the same ovary or on opposite ovaries. It is extremely rare for an embryo to split, forming twins as occurs with humans. Monitoring for possible double ovulation can be done by every-other-day sonograms (rectal ultrasound) during the heat cycle, while breeding. Any double ovulation should be noted so care will be taken to examine carefully for twins during the pregnancy check.

All bred mares should be sonogrammed for twins, even if double ovulation is not suspected. The best time to check for twins is 13 to 15 days after ovulation. At this time, the embryo will appear on an ultrasound exam as a small black sphere. The embryo will still be mobile or floating about the uterus lumen. If two are found, then one can be manipulated away form the other and manually crushed. The crushing technique is very successful and the other embryo usually survives the process. Sometimes a mare will have endometrial cysts that have the appearance of an early embryo. An ultrasound exam prior to the pregnancy check will be helpful in differentiating of the two.

After day 16, the embryos become fixed and are not moveable. If they are adjacent (touching), then it is extremely difficult to crush one without crushing the other. Often, however, when the embryos are adjacent, one will spontaneously reduce from the influence of the other. Because of this natural reduction phenomenon, we usually leave the two adjacent embryos and recheck on day 29 after ovulation. If one is present, no treatment is necessary, but if both are present, then both should be aborted with a prostaglandin shot given to the mare. The mare will come back into heat and can be bred on that cycle.

If twins are carried past day 36, then the endometrial cups will have been formed, thus stabilizing the pregnancy. At this point, if the mare aborts, she may not come back in to heat causing her to miss that breeding season entirely. Any mare found to have twins that are more than 40 days old should still be aborted to protect the mare and have her in breedable shape for the beginning of the next season. If a mare conceives twins and carries them full term, not only was the last season lost but now half of the present season has also been lost.

Unfortunately, no detection method is 100 percent accurate. The best protocol is to have your mare checked by ultrasound between day 13 and day 15, and then again before day 30. The cost in time and money for the exams will be cheap insurance for your mare and foal.


Equine Veterinary Services
Raising the Standard in Equine Health

Kenton H. Arnold, DVM
Jennifer Wickline, DVM
Caroline Zouvi, DVM
505 W. British Flying School Blvd. (FM 2578)
Terrell, Texas 75160
Clinic - 972.524.7075
Fax - 972.563.7053
Emergency - 214.794.4832
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