Case for hybrid "fetal growth curves": a population-based estimation of normal fetal size across gestational age

We hypothesized that normal fetal size would be most appropriately characterized by a hybrid cross-sectional growth chart combining preterm (< 37 weeks) ultrasound-estimated fetal weights and full-term (> or = 37 weeks) birthweights. Concurrent cross-sectional charts defining fetal size were constructed from 1,331 ultrasound-estimated fetal weights (US) (24-40 weeks) and 9,553 birthweights (BW) (24-43 weeks) from a population of singleton offspring without anomalies. Percentile ranks (10th-90th) were established for both charts. Best-fit third-order equations generated by least-squares regression analysis were compared between the two growth charts. Weight distributions (percentile ranks) were compared separately across gestational age, and for preterm and term gestation. Statistical analysis was performed by chi-square analysis where appropriate with P < .05 accepted for significance. The BW standard failed to appropriately characterize ultrasound-estimated fetal weight across gestational age. (US: < 10th%, n = 42 [3.1%]; 10-90th%, n = 978 [73.5%]; > 90th%, n = 311 [23.4%], P < .001). The characterization of both preterm and term weights was significantly different between fetal size curves. The birthweight growth curve limited the designation of fetal growth restriction (< 10th%) in the preterm ultrasound group to 1.6% of subjects examined (n = 17, P < .001). In contrast, at term the ultrasound estimation of normal fetal weight (between the 10th and 90th percentiles) included 84% of the term birthweights (P < .05). Cross-sectional sonographically defined fetal size curves provide optimal characterization of preterm fetal weight as estimated by ultrasound. In term gestation, a normative size chart derived from ultrasound estimated fetal weight appears to broaden or overrepresent the tails of the distribution. Hybrid size curves, combining preterm ultrasound estimated fetal weight and term birthweight, provide the best estimate of the normal range of fetal weight across gestational age.