The position adopted during sleep by 52 pregnant women and 31 age-matched nonpregnant control subjects was studied. The pregnant group were all beyond the 30th week of gestation. In the pregnant group, 40 (77%) adopted a left tilt, 11 (21%) a right tilt and one (2%) was supine. In the control group, eight (26%) adopted a left tilt, 10 (32%) a right tilt, 12 (39%) were supine and one (3%) was prone. Closer analysis of the position adopted by the pregnant group showed: 33 adopted a full left 90° tilt, five a left 60° tilt, two a left 30° tilt, 11 adopted a full 90° right tilt and only one was supine. There was a significant difference between the sleeping positions of the pregnant and the control groups (p < 0.001). The majority of the pregnant group adopted a sleeping position that reduced the likelihood of aortocaval compression syndrome.