Cross-country comparison of perinatal outcomes between japanese mothers in the united states and japan: a cross-sectional study.

Introduction: In the USA, studies often show that immigrants exhibit relatively favourable perinatal outcomes compared with their US-born counterparts of the same race/ethnicity, despite socioeconomic and language barriers in healthcare. However, studies with individuals in their home country are limited. Methods and results: We analysed 32 028 Japanese who gave singleton births from two data sources: the US vital statistics records (N=19 462) and the Birth and Three-Generation Cohort Study conducted by the Tohoku Medical Megabank Organization (N=12 566) between 2015 and 2017. The US data were divided into two groups based on nativity: the first generation and the second generation and later. Using Japanese women who delivered in Japan as the reference, associations between nativity and perinatal outcomes were evaluated using regression models, with analyses stratified by pre-pregnancy body mass index category. The ORs (95% CI) of preterm birth were higher among Japanese women in the USA (the first generation: 1.20 (1.15 to 1.33); the second generation and later: 1.40 (1.28 to 1.54)), as were the ORs of macrosomia (1.71 (1.48 to 1.98) and 1.98 (1.68 to 2.32), respectively). In contrast, the ORs of term or post-term low birth weight were lower among Japanese women in the USA (0.77 (0.71 to 0.84) and 0.81 (0.71 to 0.91), respectively). The association of nativity on the ORs of low birth weight differed depending on the term of birth. Conclusions: Japanese immigrants to the USA have heterogenous risk of perinatal outcomes compared with Japanese who remain in Japan. Including home country data in immigration studies may provide a more detailed understanding of the impact of immigration on their perinatal outcomes.