Veterinarians might be able to prevent abortions in some pregnant mares that show premature mammary development, Dietrich H. Volkmann, BVSc, MMedVet (Gyn), Dipl. ACT, told equine veterinarians at the 2010 Western Veterinary Conference, held Feb. 14-18 in Las Vegas, Nev.

In addition to the mammary development, horse owners might also see waxing, which occurs when the balance of hormones changes to signal labor. Before the birth of a normal foal, maternal progestagens increase and then decline sharply, triggering labor.

If there is a problem with the pregnancy and the fetus is stressed, placental progestagen production initially increases and then often declines sharply immediately prior to abortion. Hormone therapy might "trick" the mare’s body into keeping the fetus.

"By the time we see premature mammary development, estrogen has decreased and progestagen has increased," Volkmann said. "If I can make the mare ‘believe’ that these acute fluctuations in progestagen concentrations never happened, and she is carrying a viable foal, we can stop the cascade that would culminate in the abortion of the fetus."

Volkmann has had good success with this approach in mares that have conceived twins. Because the placentas of the two fetuses usually differ in size, the fetus with the smaller placenta often dies during the third trimester of gestation, when the smaller placenta cannot support the fetus.

The initial distress of the malnourished fetus (causing progestagen concentrations to rise), followed by its death (causing progestagen concentrations to fall sharply), triggers the abortion of both fe