Taiwan saw an increase in immigration during the last decade. This retrospective study investigated whether immigrant status confers a protective effect on birth outcomes and whether this effect varies across racial/ethnic subgroups in Hsin-Chu County, Taiwan. Methods: A total of 30,770 singleton birth certificates from January 1, 2002 to July 31, 2007 were analyzed using ANOVA and logistic regression. Outcomes included low birth weight (LBW, < 2500 g), high birth weight (HBW, > 4000 g), preterm birth (< 37 weeks) and stillbirth. Covariates included maternal age, year of delivery, mode of delivery, medical care institution, infant sex and congenital birth defects. Five models were designed for various analyses. The reference categories included non-immigrant, non-aboriginal Taiwanese and non-Chinese-speaking immigrants. Results: Preterm rate ranged from 6.3% among mainland Chinese to 13.5% among aboriginal Taiwanese. LBW rate ranged from 4.3% among mainland Chinese to 17.3% among aboriginal Taiwanese. HBW rate ranged from 1.2% among aboriginal Taiwanese to 3.4% among mainland Chinese. Stillbirth rate ranged from 0.2% among Indonesians to 0.7% among aboriginal Taiwanese. Adjusted odds ratio (OR) was lower among mainland Chinese (preterm OR, 0.77; LBW OR, 0.62) but higher among aboriginal Taiwanese (preterm OR, 1.79; LBW OR, 2.68; stillbirth OR, 2.92). HBW rate was significantly higher (OR, 1.84) among mainland Chinese. Chinese-speaking immigrants showed significant differences in LBW (OR, 0.57) and HBW (OR, 1.62) compared with non-Chinese-speaking immigrants. Conclusion: An epidemiologic paradox and heterogeneity of birth outcomes were observed among immigrants in this study. However, aboriginal Taiwanese constituted the subgroup with the highest risk. Further research is needed to identify the determinants of birth outcomes.