Adjusting growth standards for fetal sex improves correlation of small
babies with stillbirth and adverse perinatal outcomes: A state-wide
population study
Abstract
Objective: Identify the proportion of infants reclassified if
sex-specific birthweight charts were used, and if this reclassification
has an impact on the correlation between birthweight centile and adverse
perinatal outcome. Design: Retrospective cohort study Setting: Victoria,
Australia. Population: All infants born from 2005-2015 (529,261)
Methods: We applied GROW centiles, either adjusted or unadjusted for
fetal sex. We compared proportions of small for gestational age (SGA,
<10th centile) infants, then the populations of males
considered small only by sex-specific charts and females considered
small only by unadjusted charts. Main Outcome Measures: Stillbirth,
combined perinatal mortality, NICU admissions, Apgars <7 at 5
minutes, emergency caesarean sections. Results: Of those <10th
centile by unadjusted charts, 39.6% were male, and 60.5% female. Using
sex-specific charts, 50.3% <10th centile were male and 49.7%
female. 9,449 (19.2%) females that were SGA according to unadjusted
charts were appropriate for gestational age
(AGA,>10th-<90th centile) using sex-specific
charts. These reclassified newborn females were not at increased risk of
adverse outcomes compared with an AGA infant, but were at increased risk
of being iatrogenically delivered for suspected growth restriction (RR
4.90, 95%CI 4.39–5.48). 8,048 male infants were reclassified as SGA by
sex-specific charts (25% SGA increase). Compared with AGA infants,
these reclassified male newborns were at greater risk of stillbirth (RR
1.94, 95%CI 1.30-2.90) and all other adverse perinatal outcomes.
Conclusions: Sex-specific growth standards classify a new high-risk
cohort of male infants as SGA, and exclude a cohort of females, whose
risk is no greater than appropriately grown infants.