A total of 14,541 pregnant women resident in Avon, United Kingdom, with expected delivery dates between April 1, 1991, and December 31, 1992, were enrolled in ALSPAC, representing 85% of the eligible population.22,23 Of the 14,472 pregnancies with known birth outcomes, 13,988 infants were alive at 1 year. Mothers completed self-administered questionnaires about themselves and their child's development, environmental exposures, and health outcomes approximately every 6 months from birth to age 7 years and every year thereafter, with data available for 7,152 children at age 13. 99% of children were between 13 years 1 month and 13 years 11 months of age at the time the age 13 questionnaire was answered (full range: 12 years 10 months to 16 years 1 month). Ethical approval for the study was obtained from the ALSPAC Law and Ethics Committee and Local Research Ethics Committees. The characteristics of this population-based sample and its generalizability have been previously reported.22 Briefly, children in Avon had parents with a similar racial distribution as the general UK population (5.1% versus 6.4% nonwhite, in Avon and the entire United
