Law birth weight (LBW), due to shortened gestation and/or inadequate fetal growth. is the major determinant of infant mortality and morbidity. Despite improvements in infant mortality, them has been no reduction in LBW rates. The authors examined the relationship between 33 maternal characteristics and the increased risks of preterm (PT) delivery or small-for-gestational-age (SGA) births in 76,444 Alberta women 1994-1997. PT was associated with preexisting medical conditions, obstetrical history, and pregnancy complications. Modifiable factors such as advanced maternal age contributed only 11% to the overall PT risk. SGA births were associated with several modifiable factors, including low prepregnancy weight, maternal age, smoking, drinking, and drug dependency. These contributed to 29% and 31% of PTand term SGA births. Smoking remains an important target for intervention, having contributed to 8% of PT births and about 24% of SGA births. SGA appears to be more amenable to prevention than PT delivery.