[Fetal bioreactive oxytocin secretion and its involvement in labor initiation]

It has been well documented that the human fetus synthesizes and secretes oxytocin (OXT) actively near term. However, the mechanisms involved in the instigation and maintenance of labor are as yet poorly understood. This study was performed to investigate the dynamics of the bioreactive OXT both in maternal and fetal fluids and the populations of receptor sites in the myometrium, amnion and decidua. The bioreactive OXT, measured by a newly developed specific radioreceptor assay, was first observed in the maternal peripheral circulation at later than 29 weeks of pregnancy and was found to be at higher levels near term. In normal vaginal deliveries, umbilical arterial OXT levels were about 4 times that of elective cesarean section, and were significantly higher than either umbilical vein or maternal peripheral plasma. The binding affinity of OXT-receptor between the myometrium and amnion revealed no difference, but the binding capacity of the myometrium was 3 times that of the amnion. The binding affinity in the myometrium increased significantly at term and binding capacity both in the myometrium and in amnion showed similar increases followed by a decrease after the onset of labor. Thus, it may be assumed that human fetal OXT directly affects the myometrium and/or indirectly the amnion to participate in the instigation and maintenance of labor.