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Estrogen Levels Mediated By Placenta May Signal Labor

Shift in ratio of two types of estrogen associated with onset of labor

THURSDAY, April 2 (HealthDay News) -- A shift in the ratio of estradiol and estriol mediated by a hormone produced in the placenta may be the factor that signals the onset of labor, according to a study published online March 3 in the Journal of Clinical Endocrinology & Metabolism.

Roger Smith, M.D., Ph.D., of John Hunter Hospital in Newcastle, Australia, and colleagues conducted a study of 500 pregnant women who were followed from the time of their first antenatal visit up to birth. Maternal blood samples were tested to measure the ratio of estradiol to estriol, progesterone to estriol, and progesterone to estradiol as well as the association between concentrations of a hormone in the placenta, corticotrophin-releasing hormone (CRH), and estriol during the last month prior to onset of labor.

At the 26-week mark, the percentage daily change in levels of CRH was much higher in preterm singletons compared to their term counterparts, and there was a strong positive association between CRH and estriol concentrations in late pregnancy, the scientists discovered. The ratio of estriol to estradiol increased, while the ratio of progesterone to estriol decreased and the ratio of progesterone to estradiol did not change, the researchers report.

"The results provide a rationale for progesterone supplementation in the prevention of preterm birth by antagonizing the declining progesterone to estriol ratio that occurs in late gestation as part of the normal progression towards labor," the authors write.

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