Reproductive effects of long-term, low-dose exposure to disinfectant by-products have not been consistently documented in large populations despite the known toxicity of high exposures and the wide-spread occurrence of low concentrations in public drinking water. We investigated the effect of low-dose exposure to total trihalomethanes (TTHM) on birth weight and gestational term in New York State. All singleton live births from 1998 through 2003 in 62 counties in New York State were linked with public water supply (PWS) system boundaries based on mother's residential address on birth certificate. Using the data from public water supply system, TTHM measurements were assigned geographically and temporally to each birth record linked with PWS boundary. Individual level maternal information including mother's race, ethnicity, education, employment status, smoking, age, along with adequacy of prenatal care utilization and infant's gender was used in a logistic model to adjust for potential confounding. A small non-linear association was detected between TTHM exposure and low birth weight (<2,500 g) births (OR 1.14; 95 % confidence interval (CI) 1.08-1.21), preterm births (OR 1.14; 95 % CI 1.08-1.20) and for small for gestational age births (OR 1.10; 95 % CI 1.04-1.16) suggesting a small increase in risk for these birth outcomes with chronic low maternal exposure to drinking water containing trihalomethanes. Maternal exposure to TTHMs during pregnancy may be associated with low birth weight, preterm births and small for gestational age births.