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Oxytocin in Human Pregnancy and Parturition

From the Department of Obstetrics and Gynecology at Cornell University Medical College, New York, New York.

Abstract

Oxytocin levels in plasma and amniotic fluid were measured by a specific radioimmunoassay with a sensitivity of 5 pg/assay tube. Twenty-one of 28 pregnant patients had detectable levels of oxytocin ranging from 2.7 to 24.0 pg/ml. In 11 subjects in labor the mean ± SE of maternal venous plasma oxytocin was a) 43.1 ± 11.2 pg/ml (range: undetectable-72.5 pg/ml) in the first stage, b) 114.1 ± 20.7 pg/ml (range: 18.8–140.0 pg/ml) in the second stage, and c) 63.9 ± 10.8 pg/ml (range: 3.9–117.5 pg/ml) in the third stage. In 24 patients with normal labor and vaginal delivery, mean ± SE oxytocin concentrations were as follows: maternal venous plasma 40.6 ± 11.6 pg/ml (range: undetectable-72.5 pg/ml), umbilical arterial plasma 114.8 ± 18.3 pg/ml (range: 14.0–350 pg/ml), and umbilical venous plasma 37.5 ± 5.5 pg/ml (range: 6.3–107.5 pg/ml). Similarly, in 11 patients who had cesarean section after the onset of labor, maternal plasma oxytocin was 51.7 ± 13.1 pg/ml (range: 45.5–151.7 pg/ml); umbilical arterial plasma was 121.3 ± 29.9 pg/ml (range: 34.0–344.8 pg/ml); umbilical venous plasma oxytocin was 36.6 ± 10.1 pg/ml (range: 11.0–25.0 pg/ml); and amniotic fluid oxytocin was 43.5 ± 12.5 pg/ml (range: 11.0–96.0 pg/ml). In contrast, in 10 patients with cesarean section without labor, oxytocin level in maternal plasma was 26.7 ± 7.8 pg/ml (range: undetectable-63.5 pg/ml); in umbilical arterial plasma was 47.1 ± 7.9 pg/ml (range: 1.2–77.1 pg/ml); and in umbilical venous plasma was 27.6 ± 6.8 pg/ml (range: undetectable-60.3 pg/ml). The umbilical arteriovenous difference in oxytocin concentration was significantly higher in patients with labor than in patients without labor (P < 0.01). The results indicated 1) that maternal plasma has detectable levels of oxytocin during pregnancy which increases during labor with peak levels during the second stage, 2) a higher fetal concentration of oxytocin which flows towards the maternal compartment, and 3) that the fetus is a possible source of oxytocin for the mother during human labor.

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