The relationship between maternal cigarette smoking during pregnancy and obstetric complications is well established. Previous studies suggest a dose-response relationship with increasing risks for complications associated with increased number of cigarettes smoked and duration of smoking during pregnancy. Little data are available for the risks of adverse pregnancy outcomes among nonsmoking women following exposure to environmental tobacco smoke (ETS; secondhand smoke). A number of meta-analyses have found that ETS reduces mean birth weight and may increase the risk of low birth weight but does not seem to affect gestational age or the rate of preterm birth. There is little or no information on other adverse perinatal outcomes.This retrospective cohort study investigated the effects of ETS on perinatal outcomes in a population of nonsmoking women with singleton pregnancies. The participants were identified using the Newfoundland and Labrador Provincial Perinatal Database. Perinatal outcomes were compared between women with self-reported exposure to ETS (n ϭ 1202; 11.1%) and those with no exposure (n ϭ 10,560; 89.9%). Multiple logistic regression models were used to compare outcomes between the 2 groups, adjusting for maternal age, parity, partnered status, work status, level of education, body mass index, alcohol use, illicit drug use, and gestational age at delivery. The primary study outcome measures were birth weight, head circumference, birth length, and stillbirth. Secondary outcomes examined were prelabor rupture of membranes, gestational age at delivery (including preterm birth Ͻ37 weeks and Ͻ34 weeks of gestation), Apgar score, endotracheal intubation for resuscitation, neonatal intensive care unit admission, congenital anomalies, respiratory distress syndrome, intraventricular hemorrhage, necrotising enterocolitis, neonatal bacterial sepsis, jaundice, and neonatal metabolic abnormalities.Independent risk factors associated with exposure to ETS included lower mean birth weight (adjusted odds ratio [aOR], Ϫ53.7 g; 95% confidence interval [CI], Ϫ98.4 to Ϫ8.9 g), smaller head circumference (aOR,Ϫ0.24 cm; 95% CI, Ϫ0.39 to Ϫ0.08 cm), shorter birth length (aOR, Ϫ0.29 cm; 95% CI, Ϫ0.51 to Ϫ0.07 cm), stillbirth (aOR, 3.35; 95% CI, 1.16-9.72), trends toward preterm birth Ͻ34 weeks (aOR,1.87; 95% CI, 1.00-3.53), and neonatal sepsis (aOR, 2.96; 95% CI, 0.99-8.86); all P values were Յ0.05.These findings show an association between exposure of nonsmoking pregnant women to ETS with several adverse perinatal outcomes, including reduced birth weight, smaller head circumferences, stillbirth, and shorter birth length.