Objectives To construct UK Ethnic Birth Weight Centiles (UK-EBWC) for gestational age and cut-offs for small for gestational age (SGA) for England and Wales and to evaluate the SGA misclassification using the UK centiles.
Design Analysis of national birth data.
Participants All live singleton births in England and Wales in 2006 to 2012, as recorded by the Office for National Statistics (ONS) and birth registrations, linked with National Health Service (NHS) into Numbers for Babies (NN4B).
Main Outcome Measures Both sex-specific and ethnicity-sex-specific birth weight centiles for gestational age, and ethnicity-sex-specific SGA cut-offs. Centiles were computed using the Generalized Additive Model for Location, Scale and Shape (GAMLSS).
Results Our sex-specific centiles performed well and showed an agreement between the expected and observed number of births below the centiles. The ethnicity-sex-specific centiles for Black and Asian presented lower values compared to the White centiles. Comparisons of sex-specific and ethnicity-sex-specific centiles shows that use of sex-specific centiles increases the SGA diagnosed cases by 50% for Asian, 30% for South Asian (Indian, Pakistani and Bangladeshi) and 20% for Black ethnicity.
Conclusions The centiles show important differences between ethnic groups, in particular the 10th centile used to define SGA. To account for these differences and to minimize misclassification of SGA, we recommend the use of customized birth weight centiles.