Beyond the healthy immigrant paradox: decomposing differences in birthweight among immigrants in Spain

BACKGROUND: The healthy immigrant paradox refers to the unexpected health advantages of immigrant groups settled in host countries. In this population-based study we analyze immigrant advantages in birthweight decomposing differences between infants born to immigrant mothers from specific origins. METHOD: Using publicly available data from Spanish Vital Statistics for the period 2007–2017, differential birthweights among several groups of immigrants were estimated with an ordinary least squares regression. The Oaxaca–Blinder regression-based decomposition method was then applied to identify the extent to which differences in birthweight between groups corresponded to compositional disparities or to other factors. RESULTS: Our analysis of singleton live births to migrant mothers in Spain between 2007 and 2017 (N = 542,137) confirmed the healthy immigrant paradox for certain immigrant populations settled in Spain. Compared with infants born to mothers from high-income countries, the adjusted birthweight was higher for infants born to mothers from non-high- income European countries (33.2 g, 95% CI: 28.3–38.1, P < 0.01), mothers from African countries (52.2 g, 95% CI: 46.9–57.5, P < 0.01), and mothers from Latin American countries (57.4 g, 95% CI: 52.9–61.3, P < 0.01), but lower for infants born to mothers from Asian non-high-income countries (− 31.4 g, 95% CI: − 38.4 to − 24.3, P < 0.01). Decomposition analysis showed that when compared with infants born to mothers from high-income countries, compositional heterogeneity accounts for a substantial proportion of the difference in birthweights. For example, it accounts for 53.5% (95% CI: 24.0–29.7, P < 0.01) of the difference in birthweights for infants born to mothers from non-high-income European countries, 70.9% (95% CI: 60–66.7, P < 0.01) for those born to mothers from African countries, and 38.5% (95% CI: 26.1–29.3, P < 0.01) for those born to mothers from Latin American countries. CONCLUSIONS: Our results provide strong population-based evidence for the healthy immigrant paradox in birthweight among certain migrant groups in Spain. However, birth outcomes vary significantly depending on the origins of migrant subpopulations, meaning that not all immigrant groups are unexpectedly healthier. A significant portion of the perinatal health advantage of certain immigrant groups is only a by-product of their group composition (by age, parity, marital status, socioeconomic status, and citizenship of mother, age and migratory status of father and type of delivery) and does not necessarily correspond to other medical, environmental, or behavioral factors.