Study sample
The total source population from the MBRN between 1990 and 2016 contained data from 1,593,281 pregnancies (Fig. 1). Of these, we analysed 1,558,174 pregnancies, of which 16% (n = 245,887) were to immigrant women (foreign-born women with two foreign-born parents) and 84% (n = 1,312,287) were to non-immigrants (Norwegian-born women with at least one Norwegian-born parent). We excluded pregnancies due to missing data on maternal country of birth (518 pregnancies), pregnancies to foreign-born women with a Norwegian-born parent(s) (25,661 pregnancies) and pregnancies to Norwegian-born women with two foreign-born parents (8,928 pregnancies).