We assessed the effects of prenatal exposure to environmental tobacco smoke on fetal growth and length of gestation. The study population consisted of 389 nonsmoking women who were selected from a population-based study in southeast Finland on the basis of questionnaire information after delivery (response rate 94%). The final exposure assessment was based on nicotine concentration of maternal hair sampled after the delivery, which measures exposure during the past 2 months (i.e., the third trimester). The exposure categories were defined a priori as high (nicotine concentration [Greater and equal to] 4.00 microg/g; n = 52), medium (0.75 to < 4.00 microg/g; n = 186), and low as the reference category (< 0.75 microg/g; n = 151). In logistic regression analysis, controlling for confounding, the risk of preterm delivery (< 37 weeks) was higher in the high [adjusted odds ratio (OR) = 6.12; 95% confidence interval (CI), 1.31-28.7] and medium exposure categories (adjusted OR = 1.30; 95% CI, 0.30-5.58) compared with the reference category, and there was a 1.22 (95% CI, 1.07-1.39) increase in adjusted OR with a 1 microg/g increase in hair nicotine concentration. The corresponding adjusted OR was 1.06 (95% CI, 0.96-1.17) for low birth weight and 1.04 (95% CI, 0.92-1.19) for small-for-gestational-age.