We examined the relationships among self-reported cigarette consumption, exhaled carbon monoxide, and urinary cotinine/creatinine ratio in pregnant women. Information on these measures of smoking was collected at first and 36th week prenatal visits. Correlations between cigarette consumption and exhaled carbon monoxide were .65 at the first visit and .70 at the 36th-week visit. For urinary cotinine/creatinine ratio, the correlations were .61 and .65, respectively, at these visits. Correlations with change in cigarette consumption between the two visits were .37 for change in carbon monoxide and .33 for change in urinary cotinine/creatinine ratio. Urinary cotinine/creatinine ratio had slightly higher overall agreement with self-reported smoking status and was less likely to misclassify smokers than carbon monoxide. We conclude that urinary cotinine/creatinine ratio is the more accurate measure for validating smoking status among pregnant women, but exhaled carbon monoxide is the better measure of cigarette consumption and of changes in consumption.