Vasopressin levels during pregnancy and labor.

Baseline plasma vasopressin concentrations were measured in 10 healthy women during a normal menstrual cycle, 97 normal women during pregnancy and 43 pregnant women hospitalized during the third trimester because of pregnancy-induced hypertension (PIH). Plasma vasopressin levels were also measured in 44 normal pregnant women in early labor and in 30 parturients at delivery. The random plasma vasopressin concentrations did not vary significantly between the nonpregnant women during the follicular phase (2.3 +/- 0.2 microU/ml) and luteal phase (2.2 +/- 0.3 microU/ml) or during the third trimester in normal pregnant women (2.0 +/- 0.2 microU/ml) or those with PIH (2.0 +/- 0.1 microU/ml). There was a significant reduction (p less than 0.01) in plasma vasopressin levels in the pregnant women during the first trimester (1.5 +/- 0.1 microU/ml) and second trimester (1.5 +/- 0.1 microU/ml) as compared to levels in nonpregnant and pregnant women in the third trimester. The mean plasma vasopressin levels in the pregnant women complaining of nausea were similar to those in the pregnant women without nausea. Plasma vasopressin levels in women during labor did not increase significantly over third-trimester-pregnancy concentrations during the first stage of labor (1.9 +/- 0.1 microU/ml) or at delivery (1.8 +/- 0.1 microU/ml). These cross-sectional measurements of maternal plasma vasopressin levels do not support a role for vasopressin in the development of PIH or in the initiation or maintenance of labor.