Non-pregnant adult smokers generally exhibit fairly stable smoking behaviour over time. In studies of this population, cotinine assays are considered a 'gold standard' measure of exposure to cigarette smoke; current smoking status can be validated with high sensitivity and specificity. In contrast, there is substantial within-person fluctuation in pregnancy smoking, as women try repeatedly to quit or cut down. As a result, cotinine measures may be of limited use for validation of amount smoked, as they are informative only about recent exposure, vary with individual smoking topography and are dependent on time lapsed since the last cigarette smoked. Thus, in reproductive epidemiology, where timing, intensity and duration of exposure are critical, self-reported history of cigarette consumption may be a more relevant fetal exposure than current smoking status. If there were substantial within-person variation over the course of pregnancy, numerous measures of cotinine would be needed to characterise patterns of fetal exposure and would not be feasible in many studies. We examined self-reported smoking patterns and compared them to patterns of urinary cotinine levels in a prospective study of 998 pregnant women, recruited 1988-92. Fluctuations in smoking were considerable and, while cotinine measures and self-reported number of cigarettes were highly correlated at any given time point across women (r=0.70), the within-person correlation between the patterns of self-reported number of cigarettes and cotinine levels was weaker (r=0.33). For researchers interested in fetal outcomes in which intensity and timing of exposure are critical, we conclude that self-reported variations in smoking during pregnancy may be a valid way to characterise detailed patterns of fetal exposure in epidemiological studies.