In Reply We appreciate Chandrakumar and ‘t Jong’s comments on our study. Chandrakumar and ‘t Jong suggest that the observed association between prenatal exposure to nitric oxide (NO) and autism spectrum disorder (ASD) could be owing to childhood exposures because brain development continues after birth. We concur that pregnancy and early childhood are crucial periods of neurodevelopment. Our study focused on the prenatal period.1 Future studies are needed to investigate the role of postnatal exposures. However, it will be difficult to differentiate between prenatal and postnatal exposures that are highly associated.