A woman's life style choices before and during pregnancy have important implications for her unborn child, but information on behaviour can be unreliable when data are collected retrospectively. In particular there are no large longitudinal datasets that include information collected prospectively before pregnancy to allow accurate description of changes in behaviour into pregnancy. The Southampton Women's Survey is a longitudinal study of women in Southampton, UK, characterised when they were not pregnant and again during pregnancy. The objective of the analyses presented here is to describe the degree to which women comply with diet and life style recommendations before and during pregnancy, and changes between these time points. The analyses are based on 1490 women who delivered between 1998 and 2003 and who provided information before pregnancy and at 11 and 34 weeks' gestation. At each time point a trained research nurse ascertained smoking status and assessed food and drink consumption using a food frequency questionnaire. We derived the proportions of women who complied with recommendations not to smoke, to eat five portions of fruit and vegetables per day and to drink no more than four units of alcohol per week and 300 mg of caffeine per day. There was a notable reduction in smoking when women became pregnant: before pregnancy 27% of women smoked, whereas in early pregnancy 15% smoked. Similarly there were significant reductions in alcohol consumption and intake of caffeinated drinks: before pregnancy 54% of women drank more than four units of alcohol per week and 39% had estimated intakes of caffeine in drinks of >300 mg per day, whereas comparable figures for early pregnancy were 10% and 16% respectively. However, there was little change in fruit and vegetable intake; the percentages of women who did not achieve the recommendation to eat at least five portions of fruit and vegetables per week were 47% before pregnancy and 46% in early pregnancy. Younger women and those with fewer educational qualifications were less likely to comply with public health recommendations. Overall, 81% of women in early pregnancy complied with at least three of the recommendations. Although there is encouraging evidence of changed health behaviours in pregnancy, young women and those with few educational qualifications may particularly benefit from targeted health initiatives.