Normal growth of the fetal biparietal diameter and the abdominal diameter in a longitudinal study. An evaluation of the two parameters in predicting fetal weight.

Normal growth curves of the fetal biparietal diameter (BPD) and abdominal diameter (AD) were measured by ultrasound in 41 normal pregnant women who had had regular menstruation with a known last menstrual period. The women were studied longitudinally from the 13th - 14th week of gestation and thereafter every 2-4 weeks until spontaneous onset of labor. The BPD curve proved to be uniform, with a deviation of 4.7 days (1 SD) before the 21st week. A 3rd-degree polynomial was found to be adequate to describe the results for BPD and AD. The value of longitudinal studies is emphasized by the fact that by using the same curve with individual plateaus rather than a common curve, as most often reported in the literature, it is possible to reduce the variance for BPD and AD by 54% and 40% respectively. There was a difference of up to 7 days for the mean BPD value prior to the 24th week of gestation between the present results and previously published normal curves collected in cross-sectional investigations. When calculating the mean fetal weight on the basis of measuring the BPD and AD, we found a linear correlation between the estimated fetal weight in the 3rd trimester and the duration of pregnancy (r = 0.99, p less than 0.001). The mean percentual difference in weight between the estimated and the actual weight was 1.2% +/- 10.5% (1 SD), which is acceptable for routine clinical use.