BACKGROUND: Maternal obesity before and during pregnancy predicts maternal and infant risks of obesity and its associated metabolic conditions. Dietary and physical activity recommendations during pregnancy as well as weight monitoring are currently available in the Chilean primary health care system. However some of these recommendations are not updated and most of them are poorly implemented. We seek to assess the effectiveness of an intervention that enhances the implementation of updated nutrition health care standards (diet, physical activity, and breastfeeding promotion) during pregnancy on maternal weight gain and infant growth. DESIGN & SETTING: Cluster randomized controlled trial. The cluster units will be 12 primary health care centers from two counties (La Florida and Puente Alto) from the South-East Area of Santiago randomly allocated to: 1) enhanced nutrition health care standards (intervention group) or 2) routine care (control group). PARTICIPANTS: Women seeking prenatal care before 15 weeks of gestation, residing within a catchment area of selected health centers, and who express that they are not planning to change residence will be invited to participate in the study. Pregnant women classified as high risk according to the Chilean norms (i.e age <16 or >40 years, multiple gestation, pre-gestational medical conditions, previous pregnancy-related issues) and/or underweight will be excluded. INTERVENTION: Pregnant women who attend intervened health care centers starting at their first prenatal visit will receive advice regarding optimal weight gain during pregnancy and diet and physical activity counseling-support. Pregnant women who attend control health clinics will receive routine antenatal care according to national guidelines. We plan to recruit 200 women in each health center. Assuming a 20% loss to follow up, we expect to include 960 women per arm. MAIN OUTCOME MEASURES: 1) Achievement of adequate weight gain based on IOM 2009 recommendations and adequate glycaemic control at 24-28 weeks of pregnancy according to ADA 2011, and 2) healthy infant growth during the first year of age based on WHO standards. DISCUSSION: We expect that the intervention will benefit the participants in achieving adequate weight gain & metabolic control during pregnancy as well as adequate infant growth as a result of an increased impact of standard nutrition and health care practices. Gathered information should contribute to a better understanding of how to develop effective interventions to halt the maternal obesity epidemic and its associated co-morbidities in the Chilean population. TRIAL REGISTRATION: Clinicaltrials.gov Identifier: NCT01916603.