Salmon Bias and Preterm Birth Among Western Immigrants in China

Introduction Immigrants from Western industrialized countries are rarely found in immigrant studies. Our primary objective was to calculate the rate of cesarean delivery, 5-min Apgar score <7, and preterm birth among Chinese and Western women. Our secondary objective was to examine whether there are significant differences in terms of risk factors between Western immigrants who chose to deliver in their country of citizenship compared to those who chose to deliver in China. Methods Single-center retrospective cohort study in Shanghai, China. Multivariate logistic regression models used delivery outcome, and place of delivery (China vs. country of citizenship) as outcome variables. Results Preterm birth occurred at a rate of 3.82% among Chinese citizens, 4.12% among Chinese-born Western citizens, and 6.54% among non-Chinese-born Western citizens. After adjustment, preterm birth <37 weeks was more frequent among non-Chinese-born Western citizens compared with Chinese citizens, with an odds ratio of 1.82 (Confidence Interval 1.20-2.78), p = 0.005. Variables statistically associated with giving birth in China were maternal age ≥35 years and being Chinese-born Western, as well as the absence of medical or obstetrical conditions. Discussion Western immigrants have overall good obstetrical outcomes in China, and this could be partly explained by selective immigration, but also by the Salmon bias, as women with risk factors tend to return to their country of citizenship for the delivery. However, the preterm birth rate was higher among Western women than in their Chinese counterparts, and further research is needed.