Data sources
We used data from the Medical Birth Registry of Norway (MBRN) and Statistics Norway during 1990-2016. In brief, the MBRN comprises mandatory, standardized notification of all live- and stillbirths in Norway from 16 weeks of gestation (12 weeks of gestation from 2001). It contains information on maternal health before and during pregnancy, as well as information on maternal and infant health during labour and birth (14). Statistics Norway collects, processes and disseminates official statistics in Norway (15). The data collection includes sociodemographic and migration related factors about all individuals who are or have been resident in Norway since 1990 (16). Data from the MBRN and Statistics Norway were linked using the national identity number assigned to all residents in Norway.