Abstract 604 Epidemiology Topic Plenary, Monday, 5/3

Background: Existing fetal growth references all suffer from one or more major methodologic problems, including errors in reported gestational age (GA), biologically implausible birth weight (BW) for GA, insufficient sample sizes at low GA, single-hospital or other nonpopulation-based samples, and inadequate statistical modelling techniques.

Methods: We used the newly developed Canadian national linked file of singleton births and infant deaths for 1992-94 births, for which GA is largely based on early ultrasound estimates. We first excluded all births whose BW for GA was <-5 or >+5 SD from the mean. We then statistically modelled the coefficient of variation of BW as a function of GA using a cubic polynomial regression to estimate the SD for BW at each GA. Finally, assuming a normal distribution for BW at each GA, we used a probabilistic procedure to include vs exclude all BWs <-2 or >+2 SD from the mean, so that the number of remaining (included) births at each BW fit the expected (Gaussian) frequency.

Results: The resulting male and female curves provide improved means, z-scores, and (especially) cut-offs for defining small- and large-for-GA (SGA and LGA) births. LGA cut-offs at low GAs are considerably lower than those of existing references, while SGA cut-offs postterm are considerably higher. For example, compared with the current WHO reference from California (Williams et al 1982) and a recently proposed U.S. national reference (Alexander et al 1996), the 90th percentiles for singleton males at 26 completed weeks for the new Canadian reference is 1167 g vs 1288 g (Williams) and 1362 g (Alexander), and at 30 weeks are 1932 vs 2159 and 2710 g. The corresponding 10th percentiles at 42 weeks are 3180 vs 3086 and 2998 g, and at 43 weeks are 3244 vs 3120 and 2977 g.

Conclusions: This new sex-specific, population-based reference should improve clinical assessment of growth in individual newborns, population-based surveillance of geographic and temporal trends in BW for GA, and evaluation of clinical or public health interventions to enhance fetal growth.