Mothers completed several questionnaires at enrolment, throughout pregnancy and during their offspring's infancy and childhood. In a questionnaire completed at week 18 of gestation, they were asked to recall their alcohol consumption just before the current pregnancy, during the first trimester and in the previous 2 weeks or at the time when they first felt the baby move. Instructions specified that one drink was equivalent to 1 unit (8 g) of alcohol, and one pint of beer to 2 units. Response categories for these questions were: never, <1 unit/week, ≥1 units/week, 1–2 units/day, 3–9 units/day, ≥10 units/day. After excluding never drinkers, the last three categories were grouped together. Thus three categories were formed to create the variable ‘drinking before pregnancy’, whereas for ‘drinking during first trimester’ only two categories were formed (<1 unit/week and ≥1 units/week), after excluding those who stopped drinking. Around week 32 of gestation, women were asked about their average weekday and weekend alcohol consumption, from which weekly alcohol intake was derived. However, attrition was high (41%) and the format of this variable was different from the
