- One of the most important hormones synthesized by the placenta during pregnancy is progesterone. The regulating mechanisms of progesterone synthesis and the mechanism responsible for the spontaneous onset of labor in women are still not fully understood. Progesterone is thought to have been involved in human parturition. The objective of this study was to compare the levels of progesterone in the human placentas, at the end of the gestation (37-41 weeks) in vaginal versus cesarean deliveries, and to evaluate the pattern of progesterone accumulation, instantly following its synthesis by the human placenta at the end of the pregnancy. Progesterone levels in human placental tissue were determined by immunochemiluminescent analysis, following tissue homogenization. Progesterone secretion and accumulation pattern in the placental tissue was demonstrated using the ex vivo, closed, dual perfusion system of isolated human placental cotyledon. Immunochemiluminescent analysis of progesterone levels in human normal and cesarean-delivered placentas showed that placentas following normal vaginal delivery store higher concentrations of progesterone, and produce progesterone more intensively. Results obtained from 120-min perfusions (of vaginal and cesarean-delivered placentas) showed that progesterone tended to accumulate in the maternal rather than the fetal compartment. These data indicate that progesterone levels continuously rise till the end of pregnancy, with no apparent drop in progesterone levels during the labor process. In addition, progesterone is released from the syncytiotrophoblast preferably into the maternal component of the placental tissue.