[Causes of labor initiation in man: role of oxytocin and prostaglandins].

Questions related to the mechanism of human labor are of central clinical importance nowadays, since the majority of perinatal mortality and morbidity is due to disregulation of uterine contractility mainly premature onset of labor. During delivery of spontaneous or induced onset endogenous prostaglandin F-synthesis increases dramatically and reaches a maximum at the time of placental separation. These increased amounts of prostaglandin F and E lead to myometrial contractions and to a reduction of the cervical resistance. Post partum, prostaglandins lead to the contracture of the myometrium necessary for separation and expulsion of the placenta. The precise causes for initiation of parturition at term however have not been fully elucidated. The present review presents a theory in which oxytocin acts as central trigger of labor. At the end of human pregnancy a marked rise in the concentration of oxytocin receptors in the myometrium can be observed, thereby leading to an increased sensitivity of the myometrium towards oxytocin. Therefore, a small increase of the circulating oxytocin concentration in the maternal peripheral blood (for example through addition of fetal oxytocin) is sufficient to induce contractions. Apart from inducing contractions, oxytocin also leads to a stimulation of prostaglandin synthesis through receptors in the decidua. Prostaglandins themselves lead to further contractions, soften the cervix, induce gap-junctions and sensitize the myometrium further for oxytocin, thereby leading to a progressive cervical dilatation. At the end of the first stage of labor, the membranes usually rupture leading to a further increase in prostaglandin synthesis, so that the mechanism can no longer be interrupted exogenously.(ABSTRACT TRUNCATED AT 250 WORDS)