Impact of Maternal Age on the Foreign-Born Paradox.

To examine the effect of maternal age on the risk of preterm birth and its phenotypes in foreign-born compared to native-born mothers. The 2014-2017 US Birth and Fetal Death data were analyzed (N = 14,867,880). Log-binomial regression models were used to estimate adjusted prevalence ratios (APR), quantifying the association between mother's nativity and preterm birth and its phenotypes, stratified by maternal age. Foreign-born mothers had a 13% lower probability of preterm birth compared to their native-born counterparts (APR, 0.87 [95% CI 0.86-0.87]). This protective effect persisted across all preterm phenotypes. Stratification by maternal age showed a slightly elevated risk in preterm and moderate-to-late preterm for adolescent mothers. Our study supports the existence of the "foreign-born paradox" whereby foreign-born mothers experienced lower levels of preterm birth despite the disadvantages of living in an alien socio-cultural environment. This favorable birth outcome was present primarily in foreign-born mothers aged ≥ 20 years.