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.