While a mare with twins by her side is rare and sometimes celebrated, twinning in horses is actually dangerous for the mare, especially during delivery. More often than not, one of the twins, generally the smaller and weaker of the two, will die before birth. In fact, the chances of a mare carrying twins to term and delivering them successfully are about one in 10,000.

The manual reduction of one embryo in mares in the very early stages of carrying twins is a practice that was introduced to the breeding community in 2006 (the technique was first described in the '80s along with the increased use of the ultrasound to detect twin pregnancies), and since then it has become a common practice in managing twin cases. In a presentation given at the 2010 American Association of Equine Practitioners convention, held Dec. 4-8 in Baltimore, Md., Pete C. Sheerin, DVM, Dipl. ACT, reviewed a study he conducted on manual twin reduction. Sheerin, a reproduction specialist who carried out his study at Rood & Riddle Equine Hospital, in Lexington, Ky., examined whether the performance of the veterinarian, the drug treatment used, or the mare's age impacted the procedure's success rate.

In his retrospective approach Sheerin used the medical records of mares that underwent twin reduction at Rood & Riddle. The procedures were performed between Days 13 and 20 of gestation. Sheerin explained that mares from the same farms that had only one embryo served as controls in the study. He noted the mares' ages (which ranged from 3 to 24 years), the veterinarian that performed the procedure (14 veterinarians total), and the combination of drugs that were used on the mares (22 combinations).

Sheerin said that the mares that had undergone a twin reduction treatment had a marginally lower live foal rate (80.3%) than those mares that carried a single foal since the beginning of their pregnancy (86.7%). There was no difference in the live foal percentage of the mares that underwent a reduction from Day 13 to 16 of gestation than those undergoing the procedure between Day 17 and 20.

Although the individual veterinarian's performance affected the live foal rate, Sheerin said differences were not related to veterinary experience. He also noted that "mare populations may have impacted these results, with experienced practitioners being referred difficult cases."

Mare age also impacted twin reduction procedures. Sheerin found that mares older than 15 years of age had lower live foaling rates (66.2%) after undergoing a reduction procedure than younger mares (83.2%).

Finally, the drugs used to treat the mares after they underwent a reduction procedure seemed to impact foaling rates, said Sheerin. He explained that mares treated with flunixin meglumine (Banamine) and progesterone had foaling rates of 84%. Mares treated with other drug combinations had a live foal rate of 84%, and mares not treated with drugs had a live foal rate of 77%.

With this new research on twin reduction procedures, owners can feel confident that their mare with a twin pregnancy can undergo a successful reduction procedure and give birth to a healthy foal.

About the Author

Erica Larson, News Editor

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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