To date most studies of the impact of school-based sex education have focused either on specific, local interventions or experiences at a national level. In this paper, we use a new cross-country dataset to explore the extent to which laws on sex education affect teenage pregnancy rates in developed countries. We find some evidence that laws mandating sex education in schools are associated with higher rates of teenage fertility. Parental opt out laws may minimise adverse effects of sex education mandates for younger teens. The estimated effects of mandatory sex education are robust to some but not all of our specifications designed to tease out causality. Taken together, changes in national laws relating to sexual health are unable to explain the significant declines in teenage pregnancy rates, which have been observed in many developed countries in recent years.