Background:Prior studies have reported substantial differences in caesarean rates between migrant and non-migrant women. In this study we investigate whether the association between maternal country of birth and caesarean section is modified by length of residence in Sweden. Methods:Population-based register study. A total of 106 760 migrant and 473 881 Swedish-born women having singleton, first births were studied using multinomial multiple regression models to estimate odds ratios (OR) and 95% confidence intervals for mode of birth. Random effect meta-analyses were conducted to assess true heterogeneity between categories of length of residence. Results:Longer duration of residence was associated with an increased overall risk of both unplanned and planned caesarean section among migrant women. This pattern was more pronounced among countries grouped as having higher prevalence (compared to Swedes) of unplanned: OR≤1=1.41 (1.32-1.50); OR>1-<6=1.49 (1.42-1.57); OR6-<10=1.61 (1.50-1.72); OR≥10=1.71 (1.64-1.79) and planned caesarean section [OR≤1=1.14 (0.95-1.36); OR>1-<6=1.30 (1.13-1.51); OR6-<10=1.97 (1.64-2.37]; OR≥10=1.82 (1.67-1.98)]. The results were robust to social, obstetric and health adjustments. There were some country-of-origin-specific findings. Conclusions:The fact that the risk of unplanned and planned caesarean section tended to increase with length of residence, even with adjustment for social, obstetric and health factors, suggests that receiving country-specific factors are playing an important role in caesarean section.