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.

A New Approach to Developing Birth Weight Reference Charts: A Retrospective Observational Study.

INTRODUCTION: Birth weight reference charts based on historical infant birth weights have significant bias at preterm gestations because many preterm births are associated with abnormal growth. This study aims to determine whether more accurate birth weight charts can be constructed using data only from births that follow spontaneous onset of labour. MATERIALS AND METHODS: This study was a single-centre retrospective observational study of 115,712 singleton live births. Births were classified as spontaneous or iatrogenic. Quantile regression was used to model the relationship between gestational age, sex, labour onset, and birth weight. Comparison was made of birth weights in the spontaneous and iatrogenic cohorts by gestation, and to existing ultrasound-based charts. RESULTS: Birth weights of spontaneous and iatrogenic births were significantly different for gestational age at the median and 10th centiles. Iatrogenic preterm infants weighed less than their spontaneous preterm counterparts. Median and 10th centile birth weights derived from the spontaneous birth cohort closely approximate previous ultrasound-based curves. DISCUSSION: Iatrogenic births are more likely to be associated with pre-existing growth disturbance. Inclusion of these data has significant impact on centile charts. Birth weight charts derived from only spontaneous births may offer a more accurate reference for clinicians.