Maternal and fetal plasma oxytocin levels during pregnancy and parturition in the sheep

Abstract

To assess fetal oxytocin release in relation to maternal oxytocin, serial paired maternal (femoral artery) and fetal (aorta) plasma oxytocin were determined by a specific and sensitive radioimmunoassay during pregnancy and parturition in the sheep. The maternal oxytocin level was 29.1 ± 2.6 pg/ml (mean ± SE) but was significantly lower than fetal oxytocin levels of 45.8 ± 4.3 pg/ml (p < 0.001). There was a significant correlation between paired maternal oxytocin and fetal oxytocin levels throughout late pregnancy (r = 0.2523, p < 0.05). Maternal oxytocin levels calculated for every 5 days' gestation ranged from 15.8 ± 2.7 to 32.7 ± 5.6 pg/ml while the corresponding fetal oxytocin levels (21.6 ± 3.5 to 62.5 ± 21.6 pg/ml) were always higher (92 to 135 days) but not significantly different. However, at 136 to 142 days' gestation, fetal oxytocin levels (63.9 ± 14.1 pg/ml) were significantly higher than maternal oxytocin levels (20.1 ± 3.3 pg/ml) (p < 0.01). Based on observations in two animals, fetal oxytocin levels appeared higher than maternal oxytocin levels at parturition with an increase a few days before labor. Our findings indicate that, in the sheep, fetal oxytocin levels were usually higher than but not significantly different from maternal oxytocin levels and that fetal oxytocin levels changed in relation to labor, suggestive of fetal release of oxytocin.