Normal range curves for the intrauterine growth of the biparietal diameter
Persson, P.H.; Grennert, L.; Gennser, G.; Gullberg, B.
Acta Obstetricia et Gynecologica Scandinavica. Supplement 78: 15-20
1978
ISSN/ISBN: 0300-8835 PMID: 281094 Document Number: 138839
Normal range curves for the growth of the fetal biparietal diameter (BPD) measured by ultrasound were calculated 3 different ways. Selected women (93) were examined longitudinally with 3 week intervals from the 16th gestational week to term. The 2nd method used measurements for each week complied with only the 1st measurement from each patient. From the 20th-30th wk, 3243 BDP determinations obtained in a routine screening program were used. BPD measurements (157) from 60 women with known data of ovulation were the 3rd source. The 3 growth curves were almost identical. The daily BPD increase was 0.44 mm up to the 33rd wk which the increment declined. The data were best explained by an equation of the 3rd degree (98.9%) for the whole investigated period (16-40 wk), and before the 33rd wk, best fitted to a straight line. Reliability was assessed by the test-retest method. The error introduced by the unreliability of measurements had a SD of 0.9 mm. The mean difference between examinations performed with an interval of 1 day was equal to the BPD growth for 1 day. To assess accuracy, the ultrasonic BPD measurements were checked against caliper measurements of the fetal head in the first half of pregnancy after hysterotomy and at term after cesarean section. The mean difference was 0.44 mm and 1.3 mm, respectively (SD 1.5 and 1.3 mm). The variation of the normal curve obtained from longitudinal measurements was 3.4 mm (SD) and the variation of the normal curve obtained from longitudinal measurements was 3.4 mm (SD) and the variation of the curve from patients with known date of ovulation was 2.2 mm (SD). By comparing 1st and 3rd curves, 52% of the variance around the mean of the former curve could be explained by difference in gestational age, 11% was due to lack of accuracy, 6% to lack of reliability and 31% to individual difference.