The healthy migrant theory: variations in pregnancy outcomes among US-born migrants.

Various studies have observed that infants born to foreign-born women have better birth outcomes (lower rates of preterm, low birth weight, and infant mortality) than those delivered to US-born women. While much attention has been given to the "healthy migrant effect" as an explanation for these positive outcomes, this theory has not been examined in an internally migrant population. The purpose of this study is to examine the relationship between maternal mobility history and birth outcomes among infants born to US resident mothers of Mexican origin. The study used 1995-1999 National Center for Health Statistics (NCHS) live birth/infant death cohort files of singleton infants delivered in the US to white women of Mexican origin (n = 2,446,253). Maternal mobility history (MMH), which refers to the relationship between the maternal place of birth and the state of residence at delivery, was categorized into the four following groups: (a) foreign-born-place of birth outside the US and delivery in the US; (b) outside-region-place of birth in one US region and delivery in another US region; (c) within-region-place of birth in one US region and delivery in a different state in the same US region; and (d) within-state-place of birth and delivery in the same US state. Consistently, there is evidence to support the healthy migrant effect in an internally migrant population. Unique to this study are the findings that infants born to mothers with outside-region MMH had a lower risk of low birth weight (LBW) and small-for-gestational age (SGA) compared to those who did not move. Overall, this study provides evidence that the healthy migrant effect and its relationship to birth outcomes can be applied to an internally migrant population.