Childhood leanness is associated with an increased risk of schizophrenia, but the effects of gender, age at anthropometric measurements and age at first diagnosis on this relationship are unclear. The present study aimed at elucidating these associations.

Population-based cohort study with childhood anthropometric measures obtained annually from the age of 7 to 13 years in 253,353 Danes born 1930–1976 and followed to 31 December 2010. During this period, 4936 were registered with schizophrenia. The associations of childhood BMI with risk of schizophrenia were estimated with Cox regression models.

Childhood BMI was significantly inversely associated with risk of schizophrenia, however with different patterns among boys and girls. In boys, childhood BMI had an inverse non-linear association with schizophrenia risk dependent on age at diagnosis; in particular, a surprisingly strong association was found between leanness and later onset of schizophrenia. In girls, the risk of schizophrenia decreased linearly with increasing BMI z-score (HR: 0.93; 95% CI: 0.88–0.98). In both boys and girls, birth weight was inversely associated with later risk. In girls, but not in boys, birth weight appeared to significantly modify the associations; there was a somewhat stronger inverse association in the lowest birth weight category.

Birth weight as well as childhood BMI at ages 7 through 13 years is associated with risk of schizophrenia in both genders, but with a particular high risk of late-onset in lean boys irrespective of birth weight, and in lean girls with low birth weight. If replicated, these observations may inform preventive efforts build on schizophrenia trajectories rooted in early life.