A new biomarker of nicotine metabolism, the ratio of trans-3'-hydroxycotinine to cotinine (3-HC:C), may be an accurate estimate of nicotine exposure from tobacco. Trans-3'-hydroxycotinine and cotinine are both byproducts of nicotine that are excreted in the urine. The purpose of this study is to evaluate the effectiveness of the 3-HC:C ratio as a measure of prenatal tobacco smoke exposure between the first and third trimesters of pregnancy and to examine the use of the 3-HC:C ratio in relation to infant birth weight. In addition, changes in the 3-HC:C metabolism during pregnancy will be analyzed according to different racial and ethnic groups.