In a prospective study of 712 pregnancies, we examined associations between maternal smoking, alcohol, and caffeine consumption and fetal growth. We interviewed patients at entry into care [12.9 +/- 4.3 (standard deviation) weeks], and at 28 and 36 weeks of gestation. We found the expected reductions in adjusted birthweight among women who smoked throughout pregnancy: 168 gm [95% confidence limits (CL) = -326, -10] for low/moderate amounts (< or = 15 cigarettes per day); 288 gm (95% CL = -491, -84) for heavy smoking (> 15 cigarettes per day). We also found a decrease in birthweight (-179 gm; 95% CL = -364, 7) among smokers who reported quiting early in pregnancy. First trimester alcohol consumption (average: four drinks per week) was associated with a 155-gm reduction in fetal growth (95% CL = -324, 15), even after adjustment for number of cigarettes smoked. The association, observed with all types of alcohol consumption, was stronger among smokers (-270 gm) but was also present in nonsmokers (-115 gm). Caffeine consumption showed no relation to fetal growth, even among heavy consumers, although they were relatively few. This study implicates heavy maternal smoking at any point in pregnancy, including solely in the early months, and possibly moderate alcohol drinking as causes of low birthweight.