To study the relationships between fetal and maternal oxytocin (OT) levels and the initiation of labor in sheep, paired maternal and fetal plasma OT concentrations (microunits per ml) were measured by RIA. Samples were obtained daily from pregnant ewes and their fetuses for 5 days before spontaneous delivery and frequently during the first and second stages of labor and during the 3 h after delivery. The mean maternal plasma OT concentration during the first stage of labor was not different from that preceding labor. In contrast, the mean maternal plasma OT level during stage 2 of labor was significantly higher than the earlier baseline maternal values or the mean paired fetal concentration. There was no significant increase in the mean fetal plasma OT concentration before delivery. The newborn plasma OT concentration was elevated15 min after delivery. From these data, we conclude that in the sheep, 1) the onset of labor is not associated with increased maternal plasma OT levels, 2) cervical or vaginal disension may be the stimulus for maternal OT release during stage 2 of labor, 3) an increase in the fetal plasma OT concentration does not occur before the initiation of labor or during the course of labor, and 4) stress in the final moments of labor or in early neonatal life may be responsible for elevated cord and early neonatal plasma OT levels.
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