BACKGROUND Pregnant women often remark that their balance degrades during pregnancy; however, it appears that no studies have documented the gravida's perception of her balance nor measured direction-specific changes in balance throughout pregnancy or after delivery. METHODS Thirty women, fifteen pregnant and fifteen non-pregnant controls, were tested monthly and through 6-month postpartum. For each session, perceived degradation in sense of balance, laboratory-based balance measures, stance width, and the number of falls since the previous session were recorded. Laboratory-based balance measures, quantified by direction-specific measures of postural sway, were computed from ten 30s quiet-standing trials on a stationary force platform. Repeated-measures analysis of variance, paired t-tests, and Pearson correlations were use to examine group and time effects. RESULTS For the pregnant group, perceived balance degradation and stance width were highly correlated (r = 0.94). Both increased during pregnancy (P < or = 0.016) and dropped to near-control levels after delivery (P < or = 0.004). Compared to the control group, pregnant subjects displayed increased sway, especially in the anterior-posterior and radial directions (P < or = 0.039). Anterior-posterior sway measures strongly correlated with perceived balance (0.82 > r > 0.72) and also decreased significantly between the third trimester and postpartum (P < or = 0.029). Interestingly, medial-lateral balance measures varied little during pregnancy, but increased after delivery. Contrary to recent work suggesting fall rates of 25%, only 13% of our subjects (n = 2) fell during pregnancy. CONCLUSIONS Perceived degradation in balance during pregnancy was strongly related to increasing postural sway instability in the anterior-posterior direction. Lateral stability was maintained during pregnancy and likely accomplished by increasing stance width.