The role of estradiol and progesterone in the regulation of myometrial activity for the onset of labor.

The concentrations of estradiol (E2) and progesterone (P) were measured in peripheral plasma and myometrium from women in the 35th to 42nd week 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 two groups, one 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. The results suggest that a lower P/E2 ratio at term would facilitate the action of an, as yet unknown, physiological stimulus to the onset of labor.