Serial plasma oxytocin levels during pregnancy and labor.

Maternal plasma oxytocin levels during pregnancy and labor were measured by a sensitive and specific radio-immunoassay. The mean plasma oxytocin concentration in maternal peripheral plasma showed a gradual rise towards term, reaching 12.5 microU/ml in term subjects without labor contractions. This level was not significantly different from the mean value in the 1st stage of normal spontaneous labor (12.6 microU/ml). Brief fluctuations in plasma oxytocin levels of considerable degree, and sometimes spurt release, were observed in labor and at term, but not in the mid-trimester, in serial samples collected at short intervals (about 10 sec). In normal spontaneous labor, no obvious relation was observed between the plasma oxytocin level and the uterine contraction period, and no significant difference was found between the mean value in uterine contraction periods (12.1 microU/ml) and that in relaxation periods (11.5 microU/ml).