Maternal age at delivery was derived from date of birth, collected at enrollment. Maternal and socio-demographic variables obtained during pregnancy were categorized for analysis: family social class (based on the highest occupation among the woman and her partner and dichotomized into manual versus otherwise, using the 1991 British Office of Population and Census Statistics classification); highest level of maternal education (secondary level academic qualification—‘O’ level—or higher versus lower); smoking before the current pregnancy (yes versus no); heavy smoking before the current pregnancy (≥15 cigarettes/day versus fewer); overweight before the current pregnancy [body mass index (BMI) ≥25 versus less]. Questions on nausea and recent changes in consumption of coffee, tea, cola, alcohol and cigarettes were included in the questionnaire completed in the first trimester of gestation. On the basis of questionnaires completed by partners themselves, data were derived on partner's alcoholism (ever suffered from alcoholism versus never) and alcohol drinking habits (≥1 units/day versus less, before the pregnancy).
