Other variables
Detailed background information was extracted from the MBRN and included marital status (married/cohabiting, not married/cohabiting), maternal age at birth (years), parity (0, 1, 2, 3, ≥4), multiple pregnancies (yes/no), chronic hypertension (yes/no), and maternal smoking (1st trimester: yes, no, not reported; available from 1999 onwards). From Statistics Norway, we extracted data on maternal level of education (no education, primary school, secondary school, university/college). All variables were selected due to their possible association with either the exposure or the outcome and were treated as potential covariates in the regression models.