Stroke is a major cause of disability and death, not only in Sweden but also in Europe as a whole [1] [2] . Stroke is the result of either an obstruction in the blood flow to one part of the brain (ischemic stroke) or bleeding into and around the brain due to a ruptured artery that feeds the brain (intracerebral or subarachnoid hemorrhage). Most often (85%) strokes are ischemic [1] [2] .

In 2008, cardiovascular diseases, including stroke, accounted for approximately one fourth of all deaths within Europe, though the figure varied between countries. On average 64/100,000 women and 82/100,000 male inhabitants died from stroke that year [3] . According to the Swedish National Board of Health and Welfare, a reduction in the incidence of mortality due to cardiovascular diseases and stroke in both male and female inhabitants in Sweden between 1987 and 2012 has been reported, from 128 to 49 per 100,000 women and 352 to 113 per 100,000 men among the age group 15 - 74 [4] .

Needless to say, in the last few decades there has been an increased migration influx in Sweden that has become an important factor in balancing the aging population of Sweden, and which could also be an important factor behind possible changes in the incidence of stroke, especially among women, Swedish-born as well as migrant. Furthermore, it has previously been discussed that there are substantial differences in stroke incidence and stroke subtypes between migrants from different countries among the urban population in Sweden [5] , but to what extent this could be accounted for by exposure to various biological risk factors, as well as demographic and socio-economic factors, has not been in focus when it comes to middle-aged migrant women in Sweden, and this, thus, remains to be explored. Previous studies have shown the association between stroke and many individual behavioral risk factors, including smoking, alcohol consumption, exercise, and body mass index (BMI) [6] - [10] . Therefore, from a public health perspective, it is interesting to further study the probability of a possible increase in the prevalence of stroke, especially in migrant women, in the future.

The aim of this study was to examine the incidence of stroke in women born outside as well as in Sweden. Furthermore, the aim was to examine how socio-economic factors, smoking, and physical activity could modify the risk.

After exclusion of 130 women (0.8%) with a previous history of stroke and of another 216 (1.3%) due to missing values for country of birth (160), hypertension (22), marital status (164), BMI (25), and smoking habits (158), a total of 16,857 women, where 14,849 were born in and 2,008 outside Sweden, remained in the study. During the follow-up, 705 (4.7%) women born in Sweden and 90 (4.5%) women born outside Sweden developed stroke (p = 0.60) (Figure 1).

In Europe, socio-economic inequalities in health have been targeted as one of the most important issues to be tackled by public health policy [20] . Socio-economic status is considered as an influential predictor for health and is usually measured by determining education, income, occupation, or a combination of these dimensions [21] . Individuals with higher education and a high income tend to adopt a healthier lifestyle, which might reduce the risk of having a stroke. About 16% of the women, regardless of migration status, were treated for high blood pressure. High blood pressure is well known to be a high risk factor for stroke [22] but can be modified by increased physical activity [23] . It is assumed that there is an inverse relationship between socio-economic status and stroke, i.e., lower socioeconomic status is associated with a higher risk of stroke [22] . According to Kuper et al., the risk of stroke was significantly inversely related to years of education completed [24] . Similarly, in the present study it was shown that among the total population. Another study conducted by Li et al. also supported these findings [22] .

Migrant women with a high SES in their own country might unfortunately not be able to keep the same SES in the new country and most often slide down to a lower socio-economic group due to difficulties in being assimilated into the labor market. They experience limited access to education, employment, housing, and health care facilities. They report that they feel isolated in their homes and lack social support [25] , aspects that are part of the social determinants of health, and uncertainty about jobs and financial conditions puts extra stress in their life, which heightens the risk of developing stroke. According to Sundquist et al., both migration status and low social position are independent risk factors associated with poor health [26] . Still, in the present study we could not support any significant differences for the incidence of stroke between migrant women and Swedish women of the same socio-economic level, but belonging to a group with a low level of socio-economic status was nevertheless a risk factor in all the women.

Previous studies have shown that smoking acts as a substantial risk for the occurrence of stroke [6] [7] . In the present study it was possible to observe that smoking is indeed an important precursor for stroke. This study revealed that women who had never smoked had less probability of getting a stroke compared to ex-smokers and current smokers in both groups of Swedish-born and non-Swedish-born women. Smoking, when combined with other risk factors, such as hypertension, diabetes mellitus, and cholesterol, acts synergistically to increase the risk of CVD [25] . Similarly, in this study, the risk of stroke for current smokers was increased after adjustment for biological and socio-economic risk factors.

The present study also showed that there was a significantly increased incidence of stroke for all current smokers, both among those with a high SES and those with a low SES in Swedish-born women, and among those with a low SES in migrant women. Interestingly, the likelihood of suffering a stroke varied in both groups of low and high SES among Swedish-born women as well as migrant women. The relative risk of getting a stroke was higher for migrant women who smoked and had a low SES, as compared to Swedish-born women who smoked and had a low SES, but this was inversed after adjusting for risk factors. A possible explanation could be that in the present study migrant women had a higher SES than the Swedish-born women.

It is well known that regular physical activity is an important determinant for maintaining good health, and it does not only improve bodily functions and mental health but also assists in maintaining a healthy lifestyle. Furthermore, it has been earlier reported that moderate and high levels of physical activity reduced the risk of total, ischemic, and hemorrhagic stroke [23] . In the present study it was observed that women performing moderate/high physical activity had less risk of suffering a stroke than those who performed no/low physical activity among the total population. This finding indicates that physical activity can modify the risk of having a stroke, which is in line with the findings of the above-mentioned study.

The limitations of this study need to be highlighted. In the MDCS study, the participants were divided into only Swedish-born and non-Swedish-born women, regardless of their individual country of origin, but information about the non-Swedish participants’ countries of origin could most likely have given a better idea of the risk factors for stroke. Furthermore, many migrants were missing in the study due to the language barrier, which resulted in a low number of migrant participants. Therefore, the potential relationship between modes of recruitment and socio-demographic characteristics as well as lifestyle factors raises the question of bias. As there has been an increased influx in numbers of migrants into the city of Malmo since 2000, the results would probably have been stronger if data had been collected today. Moreover, the reliability and validity of the PA score need to be discussed.

Additionally, in the present study, in spite of the fact that all the participants were women, data on the menopause, the use of oral contraceptives, and the use of estrogen, was missing, so this could not be adjusted for. We did not relate the prevalence of smoking to treatment of blood pressure in the groups, but earlier studies have shown that smoking in combination with high blood pressure increases the risk of CVD considerably by synergy [25] .

The main strength of this cohort study is the large sample size with a long follow-up period, which in turn helps to get results with a high confidence interval. Moreover, the overall participation rate in the MDCS was 40.8 percent, with a majority of women participants (42.6 percent), and 38.3 percent men [12] . The study has, furthermore, been compared to another population-based survey in Scania, and the results were evaluated and found compatible with regard to socio-economic factors, BMI, mortality, and smoking habits [12] .

This study confirms the results of other studies reporting that women with a low SES were at a higher risk of suffering a stroke than those with a high SES. Initiatives should be taken for educating individuals with a low SES, providing information regarding different risk factors as well as encouraging them to adopt healthy life styles. Besides, it is extremely important to ensure migrant women’s access to health care facilities and inform them about the health sector organization in Sweden. Informing them about where to go in case of disease, and also about disease prevention, that it is a normal task included in the Swedish system, that is not the case in many other countries.

Smoking, high blood pressure, low socio-economic circumstances (low SES), and low physical activity were some of the potential risk factors of stroke, as has been described before. In this study, smoking migrant women were more vulnerable to stroke than the Swedish-born women. In another, earlier, study, the group has reported that migrant women often have a higher BMI and are less physically active and therefore need special consideration from a public health perspective, especially in order to reduce their tobacco consumption, lower their BMI, and increase their physical activity. This is not an easy public health task, due to differences in cultural understanding of what it means to be physically active and in perceptions about weight and overweight.

In the present study it was shown that PA can reduce the risk of stroke even in smokers. Promoting physical activity is, thus, important, and it is not only efficacious but also easy and comparatively inexpensive to implement. Furthermore, it is part of the Swedish national public health goal that stipulates the importance of increasing the level of physical activity in all of the population, including all age groups and migrants. This study contains of women between 45 and 73 years of age, and it is well known that performing physical activity is reduced with age, especially among migrant women [25] [26] . Moreover, many women born outside Sweden are unaware of the positive effects of physical activity. Studies show that they are less physically active and that they report that performing physical activity is merely for the youth [25] [26] . In this study, about a third of the total population performed low or no PA, and the figure was probably even lower at the oldest end of the age group. Increasing age is a large risk factor for stroke and as PA is protective, lives could be saved and both pain and various handicaps avoided, as well as costs for society, by targeting middle-aged and older women with health-promoting physical activity interventions.

Multidisciplinary actions, involving media, knowledge-raising activities, as well as increased possibilities for migrant women, regardless of age and cultural needs, to be active in sport classes, are needed in order to promote physical activity among the total population and not least among the vulnerable group of middle-aged migrant women.

This study confirms that all women with low socio-economic circumstances have a higher risk of stroke. No statistically significant differences were found indicating that migrant women in this population had a higher risk as compared to Swedish-born women. Still, smoking migrant women were more vulnerable to stroke compared to Swedish-born women. Physical activity was shown to be effective in reducing the risk of getting a stroke among the total population and especially among the Swedish-born women. To increase physical activity in middle-aged women, in both migrant and Swedish-born women, is a highly recommended public health strategy in order to maintain the positive trend of a decrease in the incidence of stroke.

This study was supported by “The Challenges of Migration”, the scientific collaboration organization between Malmo University, the City of Malmo, and the Region of Scania.

There is no conflict of interest.

EA wrote an earlier manuscript, RK participated in the design, did the analyses, and drafted the second version of the manuscript. EJ conceived/designed the study and critically examined and finalized the manuscript.