This study was designed to test two hypotheses. First, that the changes in arterial blood pressure, induced by a transition from a lying to a standing position, are different in early and late pregnancy. Second, that birthweight is related to the change in mean arterial blood pressure in late pregnancy such that those patients whose pressure fell on standing should have the lightest babies. Fifty-three patients were studied between the 12th to 18th week, and 41 women were between the 34th to 40th week of an accurately dated, clinically normal, singleton pregnancy. Twelve nulliparous females served as controls. Blood pressure and heart rate were measured at rest, then in the standing position and finally at rest again with a Dinamap blood pressure measuring device. The variability in the pressor response to standing was approximately three times and in heart rate response two times greater in early pregnancy and in late pregnancy, when compared to the nonpregnant controls. It should be further noted that only in late pregnancy did the heart rate fall in 10 out of 41 women on standing. A linear relationship was observed in late pregnancy between the change in mean arterial blood pressure and birthweight (r = 0.57, P less than 0.001). This linear relationship was improved to r = 0.86, when only those women (16/41), who either had a rise or fall in systolic blood pressure of more than 5 mm Hg were included. These data indicate that pregnancy increases the variability in the pressor response to standing. Moreover, birthweight was directly related to the magnitude and direction of the pressor response in late pregnancy. Finally, this relationship suggests an additional cause for unexplained cases of intrauterine growth retardation.