A sthma is a common medical complication of pregnancy, affecting 6% to 8% of women. 1,2 Both the prevalence and morbidity of asthma are increasing in the general population. 3 The literature has been inconsistent regarding the relationship between asthma and adverse pregnancy outcomes. Increased risks of low birth weight (LBW), small for gestational age (SGA), and preterm delivery have been reported with asthma. [4][5][6][7][8] Larger studies, however, have identifi ed fewer signifi cant adverse obstetric outcomes. 1,9,10 A multicenter prospective observational cohort study performed by the Maternal-Fetal Medicine Units Network evaluated the relationship between asthma severity and maternal and fetal morbidity among pregnant women with asthma and matched controls. 11 This study demonstrated a strong relationship between asthma severity and exacerbation during pregnancy, as well as associations with nonpulmonary pregnancy outcomes, including cesarean delivery, preeclampsia, gestational diabetes, and Background: The article was designed to estimate the effect of active and passive household cigarette smoke exposure on asthma severity and obstetric and neonatal outcomes in pregnant women with asthma. Methods: We used a secondary observational analysis of pregnant women with mild and moderatesevere asthma enrolled in a prospective observational cohort study of asthma in pregnancy and a randomized clinical trial (RCT) comparing inhaled beclomethasone and oral theophylline. A baseline questionnaire detailing smoking history and passive household smoke exposure was given to each patient. Smoking status was confi rmed in the RCT using cotinine levels. Data on asthma severity and obstetric and neonatal outcomes were collected and analyzed with respect to self-reported tobacco smoke exposure. Kruskal-Wallis and Pearson x 2 statistics were used to test for signifi cance. Results: A total of 2,210 women were enrolled: 1,812 in the observational study and 398 in the RCT. Four hundred and eight (18%) women reported current active smoking. Of the nonsmokers, 790 (36%) women reported passive household smoke exposure. Active smoking was associated with more total symptomatic days ( P , .001) and nights of sleep disturbance ( P , .001). Among the newborns of active smokers, there was a greater risk of small for gestational age , 10th percentile ( P , .001), and a lower mean birth weight ( P , .001). There were no differences in symptom exacerbation or outcome between nonsmokers with and without passive household cigarette smoke exposure. Conclusions: Among pregnant women with asthma, active but not passive smoking is associated with increased asthma symptoms and fetal growth abnormalities. CHEST 2010; 137(3):601-608Abbreviations: ETS 5 environmental tobacco smoke; LBW 5 low birth weight; MFMU 5 Maternal-Fetal Medicine Units Network; OR 5 odds ratio; PROM 5 premature rupture of membranes; RCT 5 randomized clinical trial; SGA 5 small for gestational age