Mare Reproductive Mystery Solved

It’s a puzzle that stumped scientists for half a century: While women and other female mammals rely on the hormone progesterone to sustain pregnancy, mares have no detectable trace of the hormone during the last half of their term. How do they carry their foals to term?

Researchers have identified a hormone that supports pregnancy in mares. Paula da Silva/

Now the puzzle has been solved, and the answer may one day lead to new therapies for preventing preterm labor in pregnant women. Researchers have identified a different hormone, dihydroprogesterone, that supports pregnancy in mares. Much of the detective work was done at the University of California, Davis, as part of doctoral research by Elizabeth L. Scholtz, DVM; researchers at Duke University, the University of Kentucky and the University of South Carolina were also involved. 

Progesterone activates receptors in the tissues of the uterus and cervix, triggering hormonal responses. In this way it stimulates the growth of the uterine lining, which is essential for establishing and sustaining pregnancy. In their study, the researchers showed for the first time that DHP activates the same receptors in horses and is just as effective in maintaining pregnancies to term. “DHP is potentially the first of a new class of naturally occurring progesterone-like hormones that could prove valuable in sustaining pregnancies and preventing preterm labor in women, as well as horses,” says Professor Alan Conley, a reproductive physiologist in the School of Veterinary Medicine at UC Davis.

This article originally appeared in the July 2014 issue of Practical Horseman.

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