Preterm Birth and Birthweight-for-Gestational Age among Immigrant Women in Denmark 1978–2007: A Nationwide Registry Study Grete S. Pedersen, a,d Laust H. Mortensen, c Mette Gerster, b Janet Rich-Edwards, d Anne-Marie N. Andersen c a Department of Epidemiology, b Department of Biostatistics, Institute of Public Health, University of Southern Denmark, Odense, c Section of Social Medicine, Department of Public Health, University of Copenhagen, Copenhagen, Denmark, and d Connors Center for Women’s Health and Gender Biology, Brigham and Women’s Hospital and Harvard Medical School, Boston, MA Abstract Background: We sought to examine whether age at immigration and length of residence were associated with preterm and small-for-gestational age (SGA) delivery among immigrant women in Denmark. Methods: We included all live singleton deliveries from Danish-born women (1 626 880) and women from the five largest immigrant groups (68936) from 1978 to 2007. Data from the Danish Medical Birth Registry were linked to: parental country of origin, length of residence and age at immigration. Linear and logistic regression models were used to estimate absolute and relative differences with Danish-born women as the reference group. Results: All immigrant groups had an increased risk of SGA delivery with the highest risk among Lebanese-, Somali- and Pakistani-born women: risk differences (RDs) and 95% confidence intervals [CI] per 1000 deliveries of 50.2 [95% CI 43.7, 56.7], 70.1 [95% CI 62.2, 77.9] and 85.7 [95% CI 78.5, 92.9]. Turkish- and Pakistani-born women had increased RDs of 1.8 [95% CI 0.5, 3.1] and 2.2 [95% CI 0.1, 4.2] for very preterm and RDs of 3.5 [95% CI 0.9, 6.1] and 10.2 [95% CI 5.9, 14.5] for moderate preterm delivery. Lebanese-born women had a decreased risk of very preterm delivery, RD of -1.9 [95% CI -3.5, -0.3] and Somali-born women a lower risk of moderate preterm delivery, RD of -7.8 [-12.0, -3.6]. No differences were seen for the remaining groups. The association with length of residence for most immigrant groups was U-shaped, with highest risks among recent and long-term residents. Conclusion: Immigration was more strongly related to SGA than to preterm delivery. Observed differences in birth outcomes varied by age at immigration and length of residency in Denmark. Keywords: preterm delivery, small-for-gestational age delivery, immigration. Two recent Danish studies have found ethnic dispari- ties within perinatal, infant and early child mortality. 1,2 Both studies looked at differences among the largest immigrant groups and found that some immigrant women experience similarly, some even better, although a large majority worse pregnancy outcomes when compared with Danish-born women. Different pre- and post-migration factors have been found to affect differences in birth outcomes. 3–5 Factors which are closely related to immigration, such as length of residence, age at immigration and immigration status, may be able to explain observed differences. The absence of specific data on these variables has been missing in the majority of previous studies. 5 Ignoring these factors may mask important differences between immigrants and non-immigrants. The Danish regis- tries contain detailed immigration data which give us the opportunity to investigate if migration factors can explain poorer pregnancy outcomes in some immi- grant subgroups. Hence, in this study we sought to examine whether age at immigration and length of residence were asso- ciated with differences in preterm birth and small-for- gestational age (SGA) delivery among immigrant women in Denmark when compared with Danish- born women. To address these questions, we con- ducted a national registry study of all live singleton deliveries to Danish-born women and women from Correspondence: Grete S. Pedersen, Department of Epidemiology, Institute of Public Health, University of Southern Denmark, JB Winsløws Vej 9, 5000 Odense C, Denmark. E-mail: gspedersen@health.sdu.dk 534 doi: 10.1111/ppe.12010 © 2012 Blackwell Publishing Ltd Paediatric and Perinatal Epidemiology, 2012, 26, 534–542