The effect of lateral tilt on maternal and fetal hemodynamic variables. We measured maternal blood pressure and heart rate, fetal heart rate, and umbilical artery velocity waveforms in 25 healthy women placed in the supine and in both right and left 5 degrees and 10 degrees lateral tilt positions. Although we found no significant difference among these variables in the various maternal positions, two of 25 women became hypotensive and symptomatic in the supine and 5 degrees tilt positions. Because we could not predict which women would become symptomatic, we recommend lateral tilt of all pregnant women during operative procedures beyond 20 weeks' gestation, including those in the lithotomy position for vaginal delivery. 