The effect of maternal bladder volume on fundal height measurements

The effect of maternal bladder volume on fundal height measurements was studied in 200 non-obese Black women between 16 and 42 weeks gestation. Fundal height measurements were obtained by the same examiner immediately before and immediately after each subject voided. Examiners were blinded to fundal height measurements and the amount of urine voided. Prevoid fundal height measurements were significantly larger than postvoid fundal height measurements. The differences between pre- and postvoid fundal height measurements varied from -2.8 to 4.6 cm (mean 0.63 cm, SD 1.26). Postvoid fundal height measurements were smaller than the prevoid measurements in 69.5% of the women, and 34.5% of the differences were greater than 1.0 cm. Women who had voided within 30 min before measurement had significantly smaller differences between pre- and postvoid fundal height measurements (n = 20, mean 0.16, SD 1.22) than women who voided more than 30 min before measurement (n = 179, mean 0.68, SD 1.27). These findings indicate that women should be instructed to void within 30 min before fundal height measurements are obtained.