Strengths and Limitations
The strengths of this study are the large sample size and the nationwide population-based design, allowing precise selection and adjustments of covariates in the regression models, detailed analysis by country of birth, as well as investigation of trends over time for placental abruption for both immigrant and non-immigrant women. However, our results should be interpreted with some caution. We did not adjust for maternal body mass index as this variable was only available from 2008 onwards. Body mass index has been associated with country of birth and is differently distributed among immigrant groups (20). As maternal weight in pregnancy might be associated with placental abruption, it is possible that our results are over- or underestimated as the body mass index distribution varies between the countries represented in the study sample. Furthermore, our study focused on foreign-born women with foreign-born parents (i.e. 1st generation immigrant women). The results can therefore not be generalized to Norwegian born women with foreign-born parents or to foreign-born women with Norwegian-born parents. Additionally, as placental abruption is a clinical diagnosis that is lacking unified diagnostic criteria across Norwegian hospitals, misclassification of placental abruption cannot be excluded.