Pregnancy hormone mystery solved

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Identification of a pregnancy-supporting hormone in horses has resolved a reproductive mystery that has puzzled scientists for decades, reports a team of researchers led by a University of California-Davis veterinary scientist.

Characterization of this newly identified hormone, dihydroprogesterone, may also lead the way to better hormone therapies for preventing preterm labor in pregnant women. The findings are reported online Feb. 18 in advance of the March 18 print edition of the Proceedings of the National Academy of Sciences.

“This work ends 50 years of speculation as to how horses sustain the last half of their pregnancies, despite the fact that the hormone progesterone is no longer detectable in blood,” said Dr. Alan J. Conley, a reproductive physiologist at the UC-Davis School of Veterinary Medicine, in a Feb. 25 university press release.

“We show for the first time that in horses this ‘new’ progestin, DHP, is equally effective as progesterone in sustaining pregnancy during the last few months,” added Dr. Conley, who is the senior author on the study.

Dr. Conley and his colleagues are hopeful that further research will lead to development of natural hormone treatments in human medicine that are free of some of the health risks associated with the synthetic drugs currently available.

The role of progesterone is so important for a successful pregnancy that reproductive biologists have widely accepted for more than 80 years that pregnancies in humans and other mammals could not be carried to term without it.

The fact that mares have no detectable concentrations of progesterone during the last half of their pregnancy has led scientists to suspect that their bodies produce another, heretofore undefined, steroidal hormone that takes the place of progesterone in supporting the pregnancy.

In this study, the researchers demonstrated in pregnant mares and through laboratory analyses that the hormone DHP is as potent as progesterone in activating the progesterone receptors of horses, triggering endometrial growth and 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 pre-term labor in women, as well as horses, and captive wildlife species,” Dr. Conley said in the release.

These studies were part of the doctoral research of lead author Dr. Elizabeth L. Scholtz; she was assisted by researchers at UC-Davis, Duke University, the University of Kentucky, and the University of South Carolina.