Inadequate weight gain in pregnancy is a major contributor to low birth weight in Colorado, where the low birth weight rate is among the highest in the nation. In 2004, the Colorado Department of Public Health and Environment implemented a population-based intervention in 9 counties, including provider training and a public media campaign, to encourage pregnant women to gain an adequate amount of weight in pregnancy as defined by the 1990 Institute of Medicine guidelines. Pregnancy Risk Assessment Monitoring System survey data were used to track weight gain in pregnancy in 1997 through 2004 (baseline), 2005 (post-intervention), and 2006 and 2007 (after the intervention had concluded). During the period immediately after the implementation of the intervention, the percentage of women delivering in the 9 study counties who gained an inadequate amount of weight during pregnancy dropped from 18.4 at baseline to 12.8 in 2005. However, this progress was reversed in 2006, when the percentage of women with inadequate weight gain rose to 19.7. Training providers to educate women about the importance of adequate weight gain in pregnancy, in conjunction with a social marketing campaign, appears to be a promising approach to addressing a major contributor to low birth weight. However, a time-limited intervention is likely to have temporary results.