ALSPAC is a prospective study recruited from all pregnancies in three Bristol-based District Health Authorities with expected dates of delivery between April 1991 and December 1992, comprising 13,971 live births surviving to 1 y of age [13,14]. Birth weights were noted from hospital records and supine length was measured using a Harpenden neonatometer (Holtain Ltd, http://www.anthropometer.com) soon after birth (median 1 d, range 0–14 d) by the ALSPAC study team. Gestation was estimated using the date of last menstrual period and confirmed by antenatal ultrasound reports; in cases of discrepancy the data were reviewed by a single experienced clinician. Mother's parity and smoking during pregnancy were recorded by a questionnaire completed during pregnancy. The mother's highest educational achievement was recorded by questionnaire as an indicator of her socioeconomic status. Ethical approval was obtained from the ALSPAC and the local research ethics committees. Signed consent for anthropometry was obtained from a parent or guardian and verbal assent was obtained from the children.
