Introduction: Although ethnically mixed couples are on the rise in industrialized countries, their health behaviors are poorly understood. We examined the associations between partner's birthplace, age at immigration, and smoking during pregnancy among foreign-born women.

Methods: Population-based register study including all pregnancies resulting in a livebirth or stillbirth in Sweden (1991-2012) with complete information on smoking and parental country of birth. We compared the prevalence of smoking during pregnancy between women in dual same-origin foreign-born unions (n = 213 111) and in mixed couples (immigrant women with a Swedish-born partner) (n = 111 866) using logistic regression. Swedish-born couples were used as a benchmark.

Results: The crude smoking rate among Swedish women whose partners were Swedish was 11%. Smoking rates of women in dual same-origin foreign-born unions varied substantially by birthplace, from 1.3% among women from Asian countries to 23.2% among those from other Nordic countries. Among immigrant groups with prevalences of pregnancy smoking higher than that of women in dual Swedish-born unions, having a Swedish-born partner was associated with lower odds of smoking (adjusted odds ratios: 0.72-0.87) but with higher odds among immigrant groups with lower prevalence (adjusted odds ratios: 1.17-5.88). These associations were stronger among women immigrating in adulthood, whose smoking rates were the lowest.

Conclusions: Swedish-born partners pull smoking rates of immigrant women toward the level of smoking of Swedish-born women, particularly among women arrived during adulthood. Consideration of a woman's and her partner's ethnic background and life stage at migration may help understand smoking patterns of immigrant women.