The purpose of this study is to evaluate, in a five-year randomized controlled trial, the efficacy of an intervention (AMP - ACTIVE MOTHERS POSTPARTUM) to increase physical activity and promote a healthy diet (decreased calorie and fat intake) for weight loss among postpartum women who were overweight or obese prior to pregnancy. The hypothesis is that the proportion of women losing at least 10% of BMI from baseline to 12 months post-intervention will be significantly greater in the AMP intervention arm than in the minimal care arm.