The Role of Estradiol and Progesterone in the Regulation of Myometrial Activity for the Onset of Labor

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Abstract
The concentrations of estradiol (E2) and progesterone (P) were measured in peripheral plasma and myometrium from women in the 35th-42nd wk of pregnancy. On the basis of the subjective symptoms and objective signs of labor, judged by tocography and cervical status, the women were divided into 2 groups, 1 in labor and the other not. There were no significant inter-group differences in the concentrations of E2 or P in either plasma or the myometrium. The plasma P/E2 ratio, however, differed significantly, being lower in the women in labor. A similar tendency was found in the P/E2 ratio in the myometrium but due to a wide variation, the inter-group difference did not attain statistical significance. A lower P/E2 ratio at term would facilitate the action of an, as yet unknown, physiological stimulus to the onset of labor.