The Healthiest Babies Possible Program (HBP) is a prenatal intervention program for the City of Toronto and is designed to reduce the incidence of low birth weight babies. This study was undertaken to assess the impact of the HBP on dietary change and birth outcome for underweight and overweight expectant mothers. Dietary change was measured by the use of food scores applied to 24-hour dietary recalls throughout the pregnancy. Three recalls collected during the early stage of the program were selected to represent early interventions; three of the results collected during the latter stage of the program represent the late stage of intervention. Birth outcome was determined by weight gain and infant birth weight. For both groups, food scores improved throughout the early intervention period and the improvements were sustained through the remainder of the program. During pregnancy the underweight women gained more weight than the overweight women but delivered infants with lower birth weights.