During pregnancy, mares experience elevated estrogen levels. Why? Well, that’s largely unknown. What researchers do know is that when mares lose a pregnancy late in gestation, their estrogen levels are quite low. So researchers from the University of Kentucky recently tried to find out what the relationship is between estrogen concentrations and pregnancy loss.

Barry Ball, DVM, PhD, Dipl. ACT, Albert G. Clay Endowed Chair in Equine Reproduction at the University of Kentucky’s Maxwell H. Gluck Equine Research Center, in Lexington, presented their results at the 2014 American Association of Equine Practitioners Convention, held Dec. 6-10 in Salt Lake City, Utah.

In their study, the team treated six mares with 500 mg of letrozole, an estrogen inhibitor, every four days from Day 240 of gestation to foaling. Six untreated mares served as controls. They evaluated the mares’ estrogen and androgen (a male sex hormone) concentrations weekly and other parameters such as fetal growth and the combined thickness of the uterus and placenta via ultrasound biweekly.

Ball said the treated mares’ androgen levels increased and their estrogen levels dropped 90% immediately after the first treatment. All six mares gave birth to healthy foals with no abnormalities.

"Gestational length was unchanged, neonatal viability was normal, but birth weights of foals born to letrozole-treated mares were reduced by 15%," Ball said.

Based on these results, he concluded that reduced estrogen in late pregnancy does not affect pregnancy outcome. Supplementing mares in late gestation with estrogen, therefore, is likely unnecessary, he a