Smoking behaviour of women before and during pregnancy was determined from questionnaires (Supplementary Material, Fig. S3). In the ALSPAC study, a questionnaire was administered in the 18th gestational week, asking about lifetime, pre-pregnancy and first-trimester smoking behaviour (whether or not the woman smoked and, for smokers, the quantity of cigarettes per day). Women were questioned again about current smoking behaviour during the 32nd week of pregnancy. In the EFSOCH study, a questionnaire was administered during the 28th gestational week, asking about lifetime, pre-pregnancy, first trimester and current smoking quantity. At each time point, the data on smoking quantity from both studies were categorized into 1–9, 10–19 and 20+ cigarettes per day. Data on known covariates of smoking cessation in pregnancy (5,6) were also collected via questionnaire: age, Townsend deprivation score (24) (EFSOCH only), occupational position, educational level (ALSPAC only), parity, partner's smoking status and age started smoking (ALSPAC only).
