Customised birthweight percentiles: does adjusting for maternal characteristics matter?

Objective The objective of this study was to determine whether the improved prediction of risk for perinatal mortality obtained with the use of a customised birthweight standard can also be obtained with the use of a non-customised but intrauterine-based standard. Design Population-based cohort study. Setting Sweden. Population Births in the Swedish Medical Birth Register between 1992 and 2001 (n = 782 303) with complete data on birthweight, gestational age, sex, maternal age, pre-pregnancy body mass index, height, parity, and ethnicity. Methods We calculated the relative risks (RRs) of stillbirth and early neonatal mortality among small-for-gestational-age (SGA) births as established by (1) a customised standard, (2) a population standard based on birthweights, and (3) a population standard based on a best estimate of intrauterine weights. Main outcome measures Stillbirth and early neonatal mortality (< 7 days). Results The RRs of stillbirth and early neonatal mortality among SGA births as classified by the intrauterine standard were similar to those among SGA births as classified by the customised standard and much higher than those among SGA births as classified by the birthweight standard. Conclusions A non-customised but intrauterine-based standard has a similar ability to predict risk for stillbirth and early neonatal mortality as a customised birthweight standard. The process of customising population weight-for-gestational-age standards to account for maternal characteristics does little to improve prediction of perinatal mortality.