Feto-maternal plasma oxytocin levels in normal and anencephalic pregnancies.

In 4 women with anencephalic fetuses, the oxytocin level was measured in the maternal plasma in the second stage of labor and in the umbilical artery (UA) and umbilical vein (UV) by radioimmunoassay (RIA). In these cases, the mean maternal OT level was 13.4 +/- 8.0 microU/ml (mean +/- S.E.), which was not significantly different from the OT levels in cases of normal spontaneous birth and elective cesarean section. The UA and UV plasma OT levels in these cases were below the detectable level (less than 2 microU/ml), whereas in normal spontaneous births, they were 18.2 +/- 4.9 microU/ml and 12.2 +/- 2.3 microU/ml, respectively. The OT level in the UA was higher than that in the UV. There was no difference between the mean OT levels in patients with, versus those without contractions, but the plasma OT levels in the second stage of labor were higher than those in the first stage. In normal pregnancies the plasma OT concentration increased during parturition. These results suggest that placental transfer of maternal plasma OT to the fetal circulation is limited, and that in cases of anencephalic fetuses, maternal labor does not necessarily require fetal OT as a trigger or for its maintenance.