Within smokers (N = 1361–2379), the rs1051730 genotype was associated with smoking quantity before pregnancy [odds ratio (OR) for heavy (10+ cigarettes/day) versus light smoking (1–9 cigarettes/day) = 1.28 (95% CI 1.12–1.47); P = 0.0004] and during the first trimester [OR = 1.30 (95% CI 1.13–1.50); P = 0.0003], but not the third trimester [OR = 1.12 (95% CI 0.96–1.32); P = 0.15] (Table 3 and Supplementary Material, Table S3). Using all 7845 subjects with data on pre-pregnancy smoking status, we found no difference in the frequency of the risk allele between smokers and non-smokers [OR for being a smoker=1.02 (95% CI 0.95–1.10); P = 0.57].
