Study population

The MCS is a population-representative birth cohort study in the United Kingdom, following the health and development of children from 19,243 families who were born in the United Kingdom between September 2000 and January 2002. Details are described elsewhere (Hansen, 2014). Of the 15,459 children who took part in MCS at age 5 (when information on ASD diagnosis was first collected), we excluded those without valid information on ASD diagnosis in all four available sweeps (i.e. age 5, 7, 11 and 14 sweeps, n = 28). Those whose parents reported a diagnosis of ASD by the age 14 sweep (n = 581) were included in this study. Data were obtained from the UK Data Archive (further information found at cls.ucl.ac.uk/cls-studies/millennium-cohort-study/). The MCS is approved by the UK National Health Service Research Ethics Committee and written consent was obtained from all participating parents at each survey. The use of anonymized data for academic purposes did not require additional ethical approval.