In the pooled data set, we assessed the association between the pre-pregnancy, first trimester and third trimester smoking quantity (cigarettes per day) and the rs1051730 polymorphism, by performing linear regression of smoking quantity level on number of risk alleles (10). We also dichotomized smoking quantity to reflect ‘light’ (1–9 cigarettes per day) and ‘heavy’ (10+ cigarettes per day) smoking, because the previous study found the largest difference in allele frequency between equivalent categories (10) We assessed the association between this variable and the number of risk alleles using logistic regression.
