Comparison of characteristics between participants with IQ measured at 8 or 15 years and those that did not have IQ measured1.

1Abbreviations; BMI, body mass index; CSE, Certificate of Secondary Education; IQ, intelligence quotient; WASI, Wechsler Abbreviated Scale of Intelligence; WISC, Wechsler Intelligence Scale for children2Statistical tests were used to compare characteristics of participants who had IQ measured at 8 or 15 years, against participants who had no IQ measurements. Continuous variables were compared by using independent t-test and categorical variables were compared by using χ2 tests.3Maternal education is reported as the highest completed level on five ordinal categories from Certificate of Secondary Education(CSE), Vocational training, O(ordinary)-level (taken by the top 25% of CSE at 15 years), A(advanced)-level (involving 2 years of study beyond O-level) and degree or higher. The CSE, O-levels and A-levels are completed at secondary school.4Social class was categorized according to maternal occupation during pregnancy, according to standard UK classifications of occupation, ranging from class I (highest), II, III-non-manual, III-manual, IV, and V (lowest)[17].5Stimulation in the home environment was measured by an adaptation of the HOME questionnaire at 18 months of age[18].