To identify genetic variants associated with birth weight, we meta-analyzed six genome-wide association (GWA) studies (n = 10,623 Europeans from pregnancy/birth cohorts) and followed up two lead signals in 13 replication studies (n = 27,591). rs900400 near LEKR1 and CCNL1 (P = 2 × 10⁻³⁵) and rs9883204 in ADCY5 (P = 7 × 10⁻¹⁵) were robustly associated with birth weight. Correlated SNPs in ADCY5 were recently implicated in regulation of glucose levels and susceptibility to type 2 diabetes, providing evidence that the well-described association between lower birth weight and subsequent type 2 diabetes^, has a genetic component, distinct from the proposed role of programming by maternal nutrition. Using data from both SNPs, we found that the 9% of Europeans carrying four birth weight–lowering alleles were, on average, 113 g (95% CI 89–137 g) lighter at birth than the 24% with zero or one alleles (P_trend = 7 × 10⁻³⁰). The impact on birth weight is similar to that of a mother smoking 4–5 cigarettes per day in the third trimester of pregnancy.