Epidemiologic studies of disinfection by-products have traditionally focused on total trihalomethane (TTHM) concentration as a surrogate for maternal exposure during pregnancy. We used birth certificate data on 196,000 infants to examine the effect of third-trimester exposures on various indices of fetal development. We examined the effect of town-average concentrations of TTHM and additional exposure metrics in relation to mean birth weight, mean gestational age, small for gestational age (SGA) infancy, and preterm delivery. Trihalomethane data (TTHM, chloroform, and bromodichloromethane) from 1995-1998 were available for 109 towns in Massachusetts. Data from 1997-1998 on haloacetic acid (total haloacetic acids, dichloroacetic acid, and trichloroacetic acid), 3-chloro-4-(dichloromethyl)-5- hydroxy-2(5H)-furanone (MX), and mutagenicity were available for a limited number of towns. We observed reductions in mean birth weight (12-18 g) for maternal trihalomethane exposures > the 90th percentile compared with those < the 50th percentile. Birth weight reductions were detected for chloroform exposures > 20 microg/L and TTHM exposures > 40 microg/L. Elevated trihalomethanes were associated with increases in gestational duration and a reduced risk of preterm delivery. We found evidence of an exposure-response effect of trihalomethanes on risk of SGA, with odds ratios (ORs) ranging from 1.09 to 1.23 for bromodichloromethane exposures > 5 microg/L. Elevated mutagenic activity was associated with SGA [OR = 1.25; 95% confidence interval (CI), 1.04 to 1.51] and mean birth weight (-27 g; 95% CI, -54 to -1). Although smaller in magnitude, our findings are consistent with previous studies reporting associations between trihalomethanes and SGA. These data also suggest a relationship between fetal development indices and mutagenic activity independent of exposure to trihalomethanes, haloacetic acids, and MX.