Migration related factors
The Global Burden of Diseases (GBD) study represents a major international effort that quantifies levels and trends in health to allow for comparison of the magnitude of different risk factors and health outcomes in the world (17). We analysed data by maternal region of birth using the seven super regions defined by the GBD: (i) Central Europe, Eastern Europe and Central Asia; (ii) High-income; (iii) Latin America and Caribbean; (iv) North Africa and Middle East; (v) South Asia; (vi) Southeast Asia, East Asia and Oceania; (vii) Sub-Saharan Africa.
Separate analyses by maternal country of birth were also performed for all countries with more than 6000 registered pregnancies (14 countries) or more than 15 registered placental abruptions (an additional 11 countries) during the study period. Immigrant women from countries with fewer abruptions or pregnancies were grouped and coded as other countries . Reason for immigration included: education, family reunion/establishment, refugee, work, unspecified/other (8). Women from the Nordic countries (Denmark, Sweden, Finland and Iceland) are permitted to move freely across the Nordic countries and were included as a separate category (Nordic immigrants ). Maternal length of residence was categorized as <1 year, 1-4 years, ≥ 5 years and was calculated as the difference between the year of birth and the year of official residence permit.