Results
The characteristics of the study samples are outlined in Table S1. Among the immigrants, women from Central Europe, Eastern Europe and Central Asia, comprised the largest group (n = 56,466), while those from Latin America and Caribbean (n = 6,451) represented the smallest group. Mean maternal age across groups ranged from 28.5 to 31.2 years.
Immigrant women born in sub-Saharan Africa had higher parity and, lower education levels, but were less likely to be smokers. In contrast, women from high income countries had the highest level of education. Non-immigrant women were more likely to be smokers during pregnancy, compared to all other groups.
The mean length of residence among immigrants ranged from 5.0 to 8.6 years. Close to 50% of the sub-Saharan immigrants were registered as refugees, which is by far the highest among all the regions of birth. More than 50% of the immigrant women born in countries defined ashigh-income were Nordic immigrants .
The overall incidence of placental abruption was 0.47% for both immigrants and non-immigrants. There was a significant decrease in placental abruption for both groups over the study period. The decrease in immigrants was from 0.68% to 0.44% while the decrease in non-immigrants was from 0.80% in 1990 to 0.34% in 2016, (Fig. S1).
Immigrant women born in the sub-Saharan African region had the highest incidence of placental abruption (0.62%), while the lowest incidence (0:39%) was found in immigrants from Central Europe, Eastern Europe or Central Asia (Fig. 2; Table 1). When compared to non-immigrants, the adjusted OR for placental abruption was 1.35 [95% CI: 1.15-1.58] for women from sub-Saharan Africa and 1.05 [0.92-1.21] for women from Central Europe, Eastern Europe or Central Asia (Table 1).
The adjusted ORs for placental abruption by maternal country of birth relative to non-immigrant women is presented in Fig. 3. A strong association was found for immigrant women born in Ethiopia (2.39 [1.67-3.41]). A higher OR was also found for Brazilian women (1.58 [0.95-2.63]), but the confidence interval was wide.
Compared with non-immigrants, we found a weak association between length of residence and placental abruption for immigrants with 1-4 years of residence in Norway (1.13 [1.03-1.24]). There was also a slightly higher OR for abruption for refugees (1.15 [0.99-1.33]).