We observed a strong association between the rs1051730 risk allele and the odds of continuing to smoke in pregnancy. Each additional copy of the risk allele conferred a 1.27-fold higher odds (95% CI 1.11–1.45) of continuing to smoke during the first trimester (P = 0.0006). The association based on third trimester smoking was very similar (P = 0.0003; Table 2 and Supplementary Material, Table S1). Adjustment for pre-pregnancy smoking quantity weakened, but did not remove the associations (Table 2). The genetic associations were little altered by adjustment for covariates of smoking cessation in pregnancy (Supplementary Material, Table S1). These variables were not themselves associated with the rs1051730 risk allele (Supplementary Material, Table S2).
