Background: This study was conducted to evaluate the risks of adverse birth outcomes including low birthweight, preterm and small-for-gestational age among teen mothers. Methods: Data were collected from the Taiwan Birth Registry between 1985 and 1997. Among a total of 1 185 597 live births born to mothers aged 24 years or less, 151 259 (12.8%) were born to teen mothers aged 19 years or less. Crude odds ratios (ORs), adjusted ORs (AORs), their 95% confidence intervals and test for linear trend of maternal age-specific AOR were estimated using logistic regression analysis. Results: After controlling for certain birth characteristics and socio-economic factors, the age-specific analysis showed significantly gradient variations in increased risks of adverse birth outcomes among births of teen mothers, where the elevated risk was lessened in older teen mothers. Teen mothers aged 16 years or less were observed to experience particularly high AORs for all adverse birth outcomes of interest. Conclusions: Not all teen mothers experienced the same magnitude of increased risk of adverse birth outcomes in Taiwan. Considering all teen mothers as the same group may have overlooked the particularly high risks of adverse birth outcomes among very young teen mothers (16 years or less), who are especially vulnerable to such elevated risks.