To examine fluctuations in women's cigarette smoking during pregnancy and implications for the design of clinical interventions for pregnant smokers and research on the effects of fetal exposure to cigarettes. We examined changes in absolute smoking status in 1426 women who reportedly smoked during their last pregnancy in the National Health Interview Survey 1991 Pregnancy and Smoking Supplement and fluctuations in amount smoked in 60 pregnant smokers in the Family Health and Development Project. In the National Health Interview Survey 1991 Pregnancy and Smoking Supplement, a substantial proportion of women exhibited a pattern of repeated cessation and relapse. In multivariable logistic regression models, having more than a high school education was significantly associated with being an intermittent versus a continuous smoker (odds ratio = 1.55, P <.01) and with successful quitting versus continuously smoking or relapsing (odds ratio = 1.74, P <.01). Fluctuations in smoking intensity in the Family Health and Development Project were also substantial and, although 48% quit or reduced their smoking upon learning of their pregnancy, over half changed smoking intensity multiple times. We conclude that smoking during pregnancy is a complex and variable behavior for many women. Simple measures of smoking may lead to under-estimation of the impact of smoking on the fetus, and brief smoking cessation interventions early in pregnancy are likely to be inadequate for many smokers during pregnancy.