Background: Recent studies suggest that the basis for adverse pregnancy outcomes is often established early in pregnancy, during organogenesis. It is therefore important to take preventive action as early as possible, preferably before pregnancy. Because most adverse pregnancy outcomes occur in women who are unaware of being at risk, we conducted a randomized controlled trial, "Parents to Be." With this study, we sought to assess the extent to which women who have participated in preconception counseling (PCC) increase their knowledge on pregnancy-related risk factors and preventive measures and change their behavior before and during pregnancy and to provide an overview of adverse pregnancy outcomes among such women. Methods: Knowledge: Women aged 18-40 who attended PCC and women who received standard care were matched on previous pregnancy, time since last pregnancy, age, country of birth, and educational achievement. They were sent a questionnaire on knowledge about pregnancy-related risk factors and preventive measures. Behavior: Data on pregnancies and outcomes were collected. Two months after pregnancy, a questionnaire was sent regarding behavior before and during pregnancy. Results: Knowledge of women who received PCC (81.5%; n = 211) exceeded that of women who did not (76.9%; n = 422). Levels of knowledge in women who were not yet pregnant after PCC were comparable to those in women who became pregnant after PCC, indicating that, even before pregnancy, PCC increased knowledge in women contemplating pregnancy. After PCC, significantly more women started using folic acid before pregnancy (adjusted odds ratio [OR], 4.93; 95% confidence interval [CI], 2.81-8.66) and reduced alcohol use during the first 3 months of pregnancy (adjusted OR, 1.79; 95% CI, 1.08-2.97). Among the group receiving standard care, about 20% of all pregnancies ended in an adverse outcome; in the group with PCC this was 16% (OR, 0.77; 95% CI, 0.48-1.22). Conclusion: After PCC, women have more knowledge about essential items. Importantly, they gained this greater knowledge before pregnancy and more women changed their behavior to reduce adverse pregnancy outcomes. © 2008 Jacobs Institute of Women's Health.