A new and improved population-based Canadian reference for birth weight for gestational age

Background. Existing fetal growth references all suffer from 1 or more majo r methodologic problems, including errors in reported gestational age, biol ogically implausible birth weight for gestational age, insufficient sample sizes at low gestational age, single-hospital or other non-population-based samples, and inadequate statistical modeling techniques. Methods. We used the newly developed Canadian national linked file of singl eton births and infant deaths for births between 1994 and 1996, for which g estational age is largely based on early ultrasound estimates. Assuming a n ormal distribution for birth weight at each gestational age, we used the ex pectation-maximization algorithm to exclude infants with gestational ages t hat were more consistent with 40-week births than with the observed gestati onal age. Distributions of birth weight at the corrected gestational ages w ere then statistically smoothed. Results. The resulting male and female curves provide smooth and biological ly plausible means, standard deviations, and percentile cutoffs for definin g small- and large-for-gestational-age births. Large-for-gestational age cu toffs (90th percentile) at low gestational ages are considerably lower than those of existing references, whereas small-for-gestational-age cutoffs (1 0th percentile) postterm are higher. For example, compared with the current World Health Organization reference from California (Williams et al, 1982) and a recently proposed US national reference (Alexander et al, 1996), the 90th percentiles for singleton males at 30 weeks are 1837 versus 2159 and 2710 g. The corresponding 10th percentiles at 42 weeks are 3233 versus 3086 and 2998 g. Conclusions. This new sex-specific, population-based reference should impro ve clinical assessment of growth in individual newborns, population-based s urveillance of geographic and temporal trends in birth weight for gestation al age, and evaluation of clinical or public health interventions to enhanc e fetal growth.