Background: While studies have attributed the favourable birth outcomes of Mexico-born mothers in the USA to a 'healthy immigrant effect' that confers protection to immigrants, a comparison of immigrants with the source population in Mexico has been lacking. We compared preterm delivery (PTD) rates of Mexicoborn immigrants who delivered in California with Mexico-born women who delivered in Mexico (WIMX) and with a subgroup who delivered in the five top immigrant sending states in Mexico. Methods: Using 2009 birth records, we selected all live-born singletons of primiparous WIMX (699 129) and immigrants in California (33 251). We examined the unadjusted and adjusted association between place of delivery and any PTD (<37 weeks gestation), including PTD subcategories (early, moderate, late), using relative risks (RR) and 95% CIs. Multivariate models controlled for demographic and health system characteristics. Results: PTD rates were higher among immigrants in California (6.7%) than WIMX (5.8%) and compared to women in the sending states (5.5%). The unadjusted risk of any PTD (RR=1.17 (1.12 to 1.22)), early/ moderate PTD (<34 weeks gestation; RR=1.27 (1.18 to 1.38)) and late PTD (34-36 weeks; RR=1.14 (1.08 to 1.19)) was higher for immigrants than for WIMX and remained higher when controlling for age, education and healthcare variables. Birth weight <1500 g was also higher among immigrants (RR=1.27 (1.14 to 1.44)). Similar patterns were observed when comparing women in the sending states. Conclusions: We found no evidence of a 'healthy immigrant effect'. Further research must assess the comparability of gestational-age data in Mexican and Californian birth certificates.

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educational status prenatal care Maternal Age health insurance human risk assessment immigration middle aged Premature Birth statistics and numerical data comparative study ethnology Mexico Insurance, Health Cross-Sectional Studies United States Young Adult cross-sectional study migrant Humans classification Adolescent California Infant, Newborn Emigrants and Immigrants female newborn pregnancy birth certificate health care adult Mexico [North America] Birth Certificates Mexican Americans cesarean section womens health Mexican American