Effects of acculturation have been thought to contribute adversely to poor reproductive health in Mexican immigrants, and a traditional Mexican orientation has been hypothesized to be protective against poor birth outcomes. A population-based cross-sectional study of 4404 births of Mexico-born and U.S.-born Mexican-American women was conducted in California in 1992 to examine the effect of language use (as a proxy measure of acculturation) on reproductive outcomes. Utilizing birth certificates, supplemental questionnaires, and 1990 U.S. Census data, the study analyzed differences in the proportions of pregnancy risk factors, low birth weight infants, and preterm deliveries in six nativity/language subgroups. Compared to U.S.-born English-speakers, U.S.-born Spanish-speakers had a higher risk profile and Mexico-born English-speakers had a lower risk profile for adverse pregnancy outcomes. After controlling for covariates, U.S.-born Spanish-speakers had the highest odds for low birth weight (OR = 1.98, 95% C.I. = 1.00, 3.93) and Mexico-born English-speakers had the lowest odds for preterm delivery (OR = 0.70, 95% C.I. = 0.35, 1.40) compared to U.S.-born English-speakers. These nativity/language differences in risk profiles and pregnancy outcomes suggest that Mexican Americans do not experience a simple negative mode of adaptation to U.S. society, but rather a complex process of positive and negative acculturation, which may be dependent on socio-economic conditions or selection factors related to immigration.