Women who take antidepressants during the first trimester are almost twice as likely to give birth to children with autistic spectrum disorders, a nested case-control study has found. The Stockholm youth cohort comprises all children aged up to 17 years resident within Stockholm County between 2001 and 2007. After exclusion of those with incomplete data, there were 4,429 cases of autistic spectrum disorder. For each case there were ten age- and sex-matched controls (n = 43,227).