Fetal head and abdominal circumferences: II. A critical re-evaluation of the relationship to menstrual age.

Dynamic image ultrasound was utilized to re-examine the changes in head and abdominal circumferences (HC, AC) with gestational age in normal fetuses. Data collected in a cross-sectional study of fetal growth were evaluated using mathematical modeling techniques. The results of this analysis indicated that a linear-cubic function optimally fits the HC data (R2 = 97.3%) while the AC data was fitted optimally by a linear function (R2 = 95.5%). The predicted values calculated from these functions were in very close agreement [mean deviation (absolute value): 1.97% (HC), 1.38% (AC)] with those calculated from average functions for HC and AC obtained in a previous longitudinal growth study. Variability was not constant throughout pregnancy for either HC or AC. With HC the normal range changed from +/-1.5 cm to +/-2.5 cm at 28 wk (menstrual age). The AC variability increased progressively with increasing menstrual age and was shown to be related to the predicted AC value. The normal range was found to be 13% of the predicted AC value throughout pregnancy. Based on these results, a new set of normal HC and AC values for the 12-40 wk (menstrual age) period is presented.