Comparison of peripheral, uterine and cord estrogen and progesterone levels in laboring and nonlaboring women at term.

Although a direct effect of steroid hormones on the initiation of labor has been shown in animals, conclusive data on human parturition are lacking. To elucidate steroid changes associated with human labor, venous serum samples were obtained at cesarean section from the maternal peripheral and uterine veins and umbilical cord vein of seven laboring and seven nonlaboring women at term. Assays of estradiol (E2), estriol (E3) and progesterone (P) revealed that: (1) there is a major concentration difference in all the steroids between peripheral and local values, (2) labor is associated with a significant rise in systemic and local E2 but no change in P, and (3) the increased production of E2 does not appear to be from a fetoplacental source. These data strongly support a modulating role for alterations in steroid hormones at the onset of human labor. The results demonstrate an increase in estrogen, rather than the classic "withdrawal", as the prime factor in E2:P ratio changes associated with labor and suggest that the source of the estrogen increase may be maternal rather than fetal.