Immunologic and biologic activity of oxytocin in midtrimester pregnancy.

Immunologic and biologic activity of oxytocin (OT) were studied by simultaneous monitoring of plasma OT measured by radioimmunoassay (RIA) and uterine contractility measured by recording of intrauterine pressures during the step-up rate of infusion of synthetic OT. The results in 11 patients with midtrimester pregnancy and step-up increase of OT infusion by 20 mU. showed a positive correlation of the increase of plasma OT and uterine contractility only at the plasma OT levels of 20 to 60 muU. per milliliter and only with hypertonic contractility but no correlation at plasma levels below and above this range. The lack of correlation of "dose and response" either prior to OT infusion or during low or very high rate of infusion suggests that other factors than merely the quantity of plasma OT are involved in determining the degree of uterine response.