Interpretation
Immigrant women born in Ethiopia had an increased incidence of placental abruption compared with non-immigrants. Ethiopia is a large country with several regions and different ethnic communities, and the incidence of abruption is known to vary by region (21). The region an Ethiopian woman comes from is not registered upon arrival in Norway and it is possible that they are more likely to come from regions with a higher background incidence of placental abruption. A higher abruption incidence may also be attributed to nutritional factors. Immigrant women from Ethiopia appears to have a higher prevalence of vitamin D deficiency (22) as well as a lower prevalence of folic acid supplement use than other immigrants (23). Both these factors have previously been linked to placental vascular pathology (24-26).
Immigrant women from sub-Saharan Africa have been reported to receive suboptimal healthcare during pregnancy compared with non-immigrants in Norway (27). They also have a higher risk of several adverse outcomes in pregnancy, including stillbirth (28), hypertension (29), pre-eclampsia (29) and gestational diabetes (30). Chronic hypertension and pre-eclampsia are well-known risk factors for placental abruption, and an increased association also for placental abruption in immigrant women from sub-Saharan Africa might therefore not be unexpected. Somewhat surprisingly though, the OR was only moderately increased for this group. Our finding is in contrast to previous studies from the United States suggesting that black women have a substantially higher OR for placental abruption compared to white women (12, 31). The discrepancy between findings may however be due to methodological differences where aggregation of women into even larger categories (black and white) may result in different outcomes. Furthermore, our study focused on 1st generation immigrant women and not on race, which further complicates comparison of study results. In addition, the different composition of the immigrant population in Norway compared to the United States and dissimilarity in various risk factors associated with abruption may further explain the difference in ORs between studies.
We found a slightly higher incidence of placental abruption in refugees. Refugees comprise a vulnerable group, often with poor health and low socioeconomic status (11) and are known to have an increased risk of preterm pre-eclampsia (9) and preterm birth (32). When addressing the different needs of immigrants we must recognize the complexity of immigration (2). For some immigrants, strong health made migration possible in the first place, yet others have been forced to flee from war or natural disasters and may be of poorer health and with history of trauma. The process of refuge may be stressful, and refugees are exposed to various health risks and are generally in more vulnerable situations (10). Maternal stress during pregnancy is a potential risk factor for abruption (33) and the challenges faced by refugee women likely add significant stress in the whole process of immigration. In addition, refugees may face structural barriers with difficulties accessing information and advice during pregnancy in the receiving country, reducing the understanding of signs, symptoms and responses to pathology (10).
Several risk factors for placental abruption seems to increase with length of residence in the receiving country (34). For instance, a previous Norwegian study found an increased incidence of pre-eclampsia with increasing length of residence in immigrant women, particularly in women arriving for family reunion or establishment (9). Moreover, a Canadian study found that the risk of pre-eclampsia/eclampsia, placental abruption and placental infarction was significantly reduced in immigrant women with less than three months in Canada compared to those with more than 5 years of residence (13), possibly due to an adoption of a less healthy lifestyle after migration (13). These findings were not confirmed in our study where we found only a weak association for abruption in immigrants with 1-4 years of residence in Norway. These difference in findings could be due to methodological differences as the Canadian study presented only crude estimates with no adjustment for covariates.
There has been a temporal decline in placental abruption for both non-immigrants and immigrants over the last decades. In our study the incidence of abruption was 0.47%, which is lower than in the USA and Canada (1) but consistent with other Nordic countries (1, 35). In the United States the abruption incidence has plateaued; (1) however, the significant decline in abruptions in our study is also seen across many European countries over the last decade (1, 35). The reason for this decline may be attributable to a change in specific risk factors for abruption, like smoking. Compared to the host population, immigrants are less likely to smoke and in Norway smoking habits have declined significantly in recent times in both non-immigrants and immigrants (36). In Sweden and the United States a change in smoking habits was associated with a decline in abruption risk (1), and this may also partly explain the decrease in abruptions in Norway.