To assess use of saliva cotinine as a quantitative objective index of smoking in pregnant women. National survey using quota sampling of pregnant women in England. Face-to-face interviews were conducted in the respondents' own homes. Respondents' homes. N = 1009 pregnant women, mean age = 27.9 years. Saliva cotinine and current smoking status. Saliva cotinines revealed under-reporting among pregnant women by about 3%. Pregnant women under-report current smoking status. The optimal cut-off point for validating smoking status among this group requires further research. The results also highlight the need for an improved understanding of the effects of pregnancy on the metabolism of nicotine.