Where available, in both Stage 1 and 2 European studies, we created within-study z scores for birth length (available from 27 studies, n = 36,084), birth head circumference (20 studies, n = 23,277), and ponderal index (calculated as birth weight/length3, 27 studies, n = 35,836). The z scores were calculated by the same method as was used for birth weight. We used linear regression to assess the association between each outcome and each of the 7 confirmed birth weight SNPs, with sex and gestational age (where available) as covariables. We combined the results across studies using fixed-effects inverse variance meta-analysis.
