The primary aim of this study is to demonstrate improved pulmonary function at 5 years of age in the offspring of pregnant smokers randomized to vitamin C (500 mg/day) versus placebo. The investigators hypothesize that vitamin C supplementation in pregnancy will block the adverse effects of maternal smoking on offspring pulmonary function measured at 5 years of age by spirometry. Impulse oscillometry (IOS) will also be used at 3-5 years of age to assess pulmonary function.