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.

Keywords:
Fetal growth reference charts, Birth weights, Iatrogenic birth, Spontaneous birth
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