Adjusting growth standards for fetal sex improves correlation of small babies with stillbirth and adverse perinatal outcomes: A state-wide population study

OBJECTIVES: Sex impacts birthweight, with male babies heavier on average. Birthweight charts are thus sex specific, but ultrasound fetal weights are often reported by sex neutral standards. We aimed to identify what proportion of infants would be re-classified as SGA if sex-specific charts were used, and if this had a measurable impact on perinatal outcomes. METHODS: Retrospective cohort study including all infants born in Victoria, Australia, from 2005–2015 (529,261 cases). We applied GROW centiles, either adjusted or not adjusted for fetal sex. We compared overall SGA populations, and the populations of males considered small by sex-specific charts only (SGA(sex-only)), and females considered small by sex-neutral charts only (SGA(unadjust-only)). RESULTS: Of those <10(th) centile by sex-neutral charts, 39.6% were male and 60.5% female, but using sex-specific charts, 50.3% were male and 49.7% female. 19.2% of SGA females were reclassified as average for gestational age (AGA) using sex-specific charts. These female newborns were not at increased risk of stillbirth, combined perinatal mortality, NICU admissions, low Apgars or emergency CS compared with an AGA infant, but were at greater risk of being iatrogenically delivered on suspicion of growth restriction. 25.0% male infants were reclassified as SGA by sex-specific charts. These male newborns, compared to the AGA(all) infant, were at greater risk of stillbirth (RR 1.94, 95%CI 1.30–2.90), combined perinatal mortality (RR 1.80, 95%CI 1.26–2.57), NICU admissions (RR 1.38, 95%CI 1.12–1.71), Apgars <7 at 5 minutes (RR 1.40, 95%CI 1.25–1.56) and emergency CS (RR 1.12, 95%CI 1.06–1.18). CONCLUSIONS: Use of growth centiles not adjusted for fetal sex disproportionately classifies female infants as SGA, increasing their risk of unnecessary intervention, and fails to identify a cohort of male infants at increased risk of adverse outcomes, including stillbirth. Sex-specific charts may help inform decisions and improve outcomes.