This study used the data from a retrospective cross-sectional survey to describe the prevalence of smoking in pregnancy in a large UK sample, over more than 20 years of pregnancies. The main objective was to determine whether women in their second or subsequent pregnancy are more or less likely than primigravidae to change their smoking behaviour. The participants were 7506 ever-pregnant women, from 7702 who answered the second stage of a survey of reproductive history in 2001-02, sent to 10 828 women, aged 18-55 years on the UK electoral register. These women gave detailed information on their last pregnancy. The main outcome measure was self-reported cigarette consumption in the first trimester of pregnancy. Nineteen per cent (1417/7506) of women in this sample smoked in the first trimester of the reported pregnancy. The data showed that there had been a decrease over the last 20 years in smoking prevalence, but this trend was not statistically significant after adjustment for confounding (P(trend) = 0.07). There was evidence that multigravidae were more likely to smoke in their pregnancies than were primigravidae (adjusted OR = 1.24, [95% CI 1.01, 1.53]), with a highly significant trend with increasing pregnancy order (P(trend) < 0.0001). Over time, women who reported smoking before pregnancy (n = 1926), showed a decreasing trend in continuing to smoke the same amount after recognising their pregnancies (P(trend) < 0.0001). After adjustment, multigravid smokers were almost 75% more likely than primigravid smokers to continue to smoke with no change in consumption (adjusted OR = 1.74, [95% CI 1.32, 2.28]), with a clear trend found with increasing pregnancy order (P(trend) = 0.001). In this UK population-based study, multigravidae were more likely to smoke in pregnancy than women pregnant for the first time. If already a smoker, they were much less likely to cut down their smoking once they found out they were pregnant, regardless of age. This suggests that a great deal of the burden of morbidity associated with smoking in pregnancy is in the multigravid group.