New Australian Birthweight Centiles

Objectives: To prepare more accurate population-based Australian birthweight centile charts by using the most recent population data available and by excluding pre-term deliveries by obstetric intervention of small for gestational age babies.

Design: Population-based retrospective observational study.

Setting: Australian Institute of Health and Welfare National Perinatal Data Collection.

Participants: All singleton births in Australia of 23-42 completed weeks' gestation and with spontaneous onset of labour, 2004-2013. Births initiated by obstetric intervention were excluded to minimise the influence of decisions to deliver small for gestational age babies before term.

Main Outcome Measures: Birthweight centile curves, by gestational age and sex.

Results: Gestational age, birthweight, sex, and labour onset data were available for 2 807 051 singleton live births; onset of labour was spontaneous for 1 582 137 births (56.4%). At pre-term gestational ages, the 10th centile was higher than the corresponding centile in previous Australian birthweight charts based upon all births.

Conclusion: Current birthweight centile charts probably underestimate the incidence of intra-uterine growth restriction because obstetric interventions for delivering pre-term small for gestational age babies depress the curves at earlier gestational ages. Our curves circumvent this problem by excluding intervention-initiated births; they also incorporate more recent population data. These updated centile curves could facilitate more accurate diagnosis of small for gestational age babies in Australia.