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.