OBJECTIVE To examine the relation of cigarette consumption and exhaled carbon monoxide levels during pregnancy and to assess the effect of these smoking measures on birth weight. METHODS Cigarette consumption and exhaled carbon monoxide levels were recorded at the first prenatal visit and the 36-week visit from women who smoked early in pregnancy. Analysis of variance was used to compare birth weights for differing levels of cigarette consumption and exhaled carbon monoxide. Correlation and regression analyses were used to estimate the effects of the smoking measures at both prenatal visits on birth weight. RESULTS Cigarette consumption and exhaled carbon monoxide levels at both visits were associated significantly with birth weight. After the first prenatal visit, a reduction in cigarette consumption of at least nine cigarettes per day or in exhaled carbon monoxide of 8 parts per million (ppm) was associated with gains in birth weight of 100 g or more. The proportion of low birth weight (LBW) infants increased significantly with increasing levels of cigarette consumption and with increasing concentrations of exhaled carbon monoxide. CONCLUSIONS Substantial reductions in cigarette consumption or in exhaled carbon monoxide levels after the first prenatal visit are needed to achieve gains in birth weight. Not smoking, or having an exhaled carbon monoxide level less than 5 ppm minimizes the likelihood of having an LBW infant.