[Initiation of labor and the fetal oxytocin contribution (author's transl)].

Abstract

This study was designed to evaluate the fetal oxytocin (OT) contribution to the instigation and maintenance of human labor. Plasma samples together with placental tissues and myometria were collected in an effort to determine immunoreactive OT concentrations in plasma, oxytocinase activities in placenta and 3H-labeled OT uptake in myometria respectively and the following results were obtained. OT concentrations in the umbilical arterial plasma (UA) at 20 to 24 weeks were doubled at 37 to 42 weeks suggesting the increase of this hormone to occur in the latter half of pregnancy. The UA values at term were significantly higher than that of maternal peripheral and umbilical venous plasma (MPV and UV). OT concentrations in UA and UV showed a linear correlation but not with MPV. The study of labor (+) and labor (-) subjects were performed in cesarean sections as well as in vaginal deliveries. The OT concentrations in UA was higher in cesarean sections with labor pains than without labor pains. In vaginal deliveries, the OT concentrations were much higher. The uptake of biologically active OT estimated by 3H-labeled OT uptake of 20,000 xg precipitate fraction of nonpregnant, first trimester and term myometria were 10.5 plus or minus 3.1, 18.3 plus or minus 6.6 and 35.4 plus or minus 6.6% respectively. While no significant changes of placental oxytocinase activities were found between these specimens. The activities seemed to be lower in cases without labor pain than those with labor pains. These findings suggest that there is OT release from the fetus during labor to participate in the instigation and maintenance of labor with increase of the uterine sensitivity to OT, however it appears to be more active in maintaining labor than in initiating it.