Updated birth weight centiles for England and Wales.

OBJECTIVES: Construct updated birth weight-for-gestational age centile charts for use in the UK and compare these to the currently used UK-WHO charts. DESIGN: Secondary analysis of national birth data. PARTICIPANTS: Centiles were constructed using 1 269 403 singleton births occurring in England and Wales in 2013-2014 as part of the MBRRACE-UK national perinatal surveillance programme. These were then validated using 642 737 singleton births occurring in England and Wales in 2015. MAIN OUTCOME MEASURES: Sex-specific birth weight-for-gestational age centiles. Centiles were created using the lambda-mu-sigma method via the GAMLSS package in R. This method transforms the skewed birth weight distribution to approximate a normal distribution, allowing any birth weight centile to be produced. RESULTS: The new centiles performed well in the validation sample, with the observed and expected proportion of births below a given centile in agreement. Overall, driven by the predominance of term births, the UK-WHO charts classify a smaller proportion of infants as below a given centile. For example, the UK-WHO estimates classified only 1.32% (8035/606 430) of term infants born in 2015 as below the second centile, compared with 1.97% (11 975/606 430) using the new MBRRACE-UK centiles. At the earliest gestational ages, however, the opposite is observed, with the UK-WHO classifying a larger proportion of infants as below a given centile, particularly at the lower end of the birthweight distribution. CONCLUSIONS: We have constructed and validated updated birth weight-for-gestational age centiles using a contemporary sample of births occurring in England and Wales. The benefits of these updated centiles will be first to assist the national surveillance of perinatal mortality programme by improving the identification of the proportion of stillbirths and neonatal deaths affected by intrauterine growth restriction and, second, to aid clinicians by more accurately identifying babies who require increased monitoring in the period immediately following birth.