Objectives: To examine the prevalence of positive lifestyle behaviours before and during pregnancy in Ireland. Design: Cross-sectional study. Setting: Population-based study in Ireland. Participants: A total of 718 women of predominantly Caucasian origin from the Pregnancy Risk Assessment Monitoring System (PRAMS), Ireland, were included. Primary and secondary outcome measures: Positive lifestyle behaviour changes before and during pregnancy in Ireland on alcohol consumption, smoking, folate use and nutrition. Results: Of 1212 women surveyed, 718 (59%) responded. 26% were adherent to all three recommendations on alcohol consumption, smoking and folate use before pregnancy. This increased to 39% for the same three behaviours during pregnancy, with greater increases in adherence observed among women with the lowest adherence before pregnancy. Age, education and ethnicity gaps in adherence before pregnancy appeared to narrow during pregnancy. Adherence to all seven food pyramid guidelines was less than 1% overall, and less than 1% of participants met all four micronutrient guidelines on vitamin D, folate, calcium and iron intake around the time of pregnancy. Conclusions: Low levels of healthy lifestyle behaviours before pregnancy and low levels of positive lifestyle behaviours during pregnancy demonstrate an urgent need for increased clinical and public health efforts to target deleterious health behaviours before, during and after pregnancy.