Fetal size classified using gestational days rather than gestational weeks improves correlation with stillbirth risk: A statewide population study.

Abstract

ObjectiveMany growth charts provide single centile cutoffs for each week of gestation, yet fetuses gain weight throughout the week. We aimed to assess whether using a single centile per week distorts the proportion of infants classified as small and their risk of stillbirth across the week.DesignRetrospective cohort study.SettingVictoria, Australia.PopulationSingleton, non-anomalous infants born from 2005-2015 (529,261).MethodsWe applied growth charts to identify small-for-gestational-age (SGA) fetuses on week-based charts (single centile per gestational week) and day-based charts (centile per gestational day).Main outcome measuresProportions ResultsUsing week-based charts, 12.1% of infants born on the first day of a gestational week were SGA, but only 7.8% on the final day; ie. an infant born at the end of the week was 44% less likely to be classed as SGA (pConclusionsGrowth standards using a single cutoff for a gestational week overestimate the proportion of infants that are small at the beginning of the week and underestimate the proportion at the end. This distorts the risk of stillbirth amongst SGA infants based on when in the week an infant is born. Day-based charts should be used.