Time of Twin Reduction Affects Live Foaling Rate

Twin pregnancies put mares at risk and rarely result in healthy foals, which explains why one twin is typically reduced by manual crushing early in gestation. Sometimes, twin pregnancies go undetected in early gestation and are not identified until later. Reducing a twin later in pregnancy is a more intricate procedure. One reduction technique is called transvaginal ultrasound-guided aspiration (TUA) and according to a recent study is associated with variable but acceptable success.^*

To perform this procedure, an ultrasound probe is inserted rectally, and the embryos are visualized within the uterus. A needle is then advanced transvaginally and passed through the vaginal and uterine walls into the embryonic vesicles. The contents of the vesicles are aspirated, rendering the embryo nonviable.

To better understand factors that affect the success of TUA, equine reproduction experts reviewed medical records from mares that underwent this procedure between 2010 and 2019. TUA was performed in 381 mares with twin pregnancies diagnosed between days 21 and 82 of gestation. Five to seven days after TUA, 218 were diagnosed as pregnant with a single, viable embryo (57%). Just over one-quarter of the mares (103) were no longer pregnant, and 60 were still pregnant with twins (16%).

However, the day of gestation that TUA was performed greatly affected the outcome. For example, 65% of mares that had TUA performed on days 25 to 29 of gestation delivered a single, viable foal. When performed on days 25 to 35 of gestation, only 49% of mares delivered a live, singleton foal.

“The rate of live births of a single foal declined with increasing days of gestation. By days 60 to 82 of gestation, no live foals were produced following TUA,” explained Kathleen Crandell, Ph.D., of Kentucky Equine Research.

Other than the day of gestation on which TUA was performed, no other factors affected the number of live births. This included parity, mare age, TUA technique (e.g., duration of procedure), or sedation and other medications used during the procedure.

“These data tell us that acceptable pregnancy and foaling rates can be achieved following TUA when the procedure is performed before 40 days of gestation,” Crandell shared. “After 40 days, alternate procedures should be used for reducing one of the twins.”

In any pregnant mare, maximizing health improves foaling rates, which includes optimizing body condition during gestation.

“An excessive intake of calories during gestation can be detrimental to the mare’s health and pregnancy success. Broodmares that have a body condition score of over 6 (on a scale of 1 to 9) can be transitioned to a balanced diet by offering appropriate amounts of forage as well as a ration balancer to fill in the nutrients that are low or missing in forage without excessive calories,” said Crandell, an equine nutritionist.

She added, “In addition, supplemental omega fatty acids, specifically DHA and EPA, have a positive influence on uterine health to help with conception and pregnancy rates.” EO-3 is a palatable, potent marine-derived oil that can be top-dressed onto the mare’s feed.

 ^*Sielhorst, J., S. Baade, K.-C. Neudeck, A. Tönissen, K. Rohn, F. Hollinshead, and H. Sieme. 2024. Success rates and factors influencing pregnancy outcome after 464 transvaginal ultrasound-guided twin reductions in the mare. Equine Veterinary Journal:14071.