To determine the localization and concentrations of estrogen receptor alpha and progesterone receptors A and B in the lower uterine segment during term parturition.Biopsies were taken from 70 patients during nonelective cesarean delivery. The patients were at different stages of cervical dilatation (6 cm) and different duration of labor (6-12 hours, >12 hours). The receptor concentrations were determined with solid phase immunoassays, and their localization was investigated immunohistochemically.Estrogen receptor alpha concentration decreased significantly from 2.12 fmol/mg protein at less than 2 cm dilatation to 1.08 fmol/mg (4-6 cm) but tended to increase at greater than 6 cm. Progesterone receptor A and B concentration was 84.7 fmol/mg at less than 2 cm dilatation, decreased significantly to 36.6 fmol/mg (2-3.9 cm), and increased again with further dilation. Concentrations of both receptors did not depend on duration of labor. By immunohistochemistry only progesterone receptor A and B was found to be expressed by endothelial and smooth muscle cells of the vessels, stromal fibroblasts, smooth muscle cells in the myometrium, and glandular epithelial cells. Regardless of the extent of cervical dilatation, expression of progesterone receptors A and B was marked.A decrease in estrogen receptor alpha and progesterone receptor A and B concentration in the early phase of first stage labor may play a role in cervical dilation at term.