The Avon Longitudinal Study of Parents and Children (ALSPAC) (http://www.alspac.bris.ac.uk) is an ongoing population-based study investigating the effect of a wide range of environmental and other influences on the health and development of children [16]. Pregnant women resident in the former Avon Health Authority (which included the city of Bristol), in south-west England, who had an estimated date of delivery between 1 April 1991 and 31 December 1992, were invited to take part, resulting in a cohort of 14 541 pregnancies, 13,796 of whom were singletons or first-born of twins, and who were alive at one year of age. The current manuscript is focussed on the remaining offspring who received the questionnaire aged 16 (n = 9,384), that is they were not lost to follow up. Additional file 1: Appendix A shows the numbers present at each stage. The former County of Avon includes both urban and rural areas, and the extent to which the cohort is representative of children in the UK is discussed elsewhere [16]. The parents have completed regular postal questionnaires from pregnancy onwards, and the children have completed questionnaires from approximately 5 years.
