Association of total caffeine consumption (mg) with potential confounders.

Housing tenure was coded as: bought/mortgaged/owned with no mortgage to pay, rented from private landlord, rented from council/housing association. Crowding index was coded as number of people living in household divided by the number of rooms. Highest educational level was coded as the equivalent of: none, vocational, school to age 16, school to age 18, degree or higher. Alcohol consumption was measured in drinks per week. Tobacco consumption was measured in times per day. Linearity was imposed on the categorical variables (housing tenure, educational level). Measures of alcohol and tobacco consumption shown were taken at 18 weeks gestation, but results were similar at the other time points.