This research examines the association between parity and body weight and how this relationship is modified by sociodemographic and behavioral factors. Using multiple linear regression analysis, the study assessed the relationship between parity and relative body weight (as Body Mass Index, BMI) and how this relationship interacts with seven sociodemographic and seven behavioral factors in a national sample of 5,707 women from the Second National Health and Nutritional Examination (NHANES II) survey. After adjusting for sociodemographic factors, the amount of weight associated with parity averaged about 0.5 kg per child. However, parity-associated weight differed by sociodemographic and behavioral factors, and was much larger in some subgroups. Among 18-45 year olds, the amount of weight associated with parity was greater in blacks than in whites, less in employed than unemployed white women but greater in employed than unemployed black women, less in smokers than nonsmokers, less in those with a high level of recreational exercise, and differed with the level of nonrecreational physical activity depending on race. Among 46-74 year olds, the amount of weight associated with parity was greater in married than unmarried women, and less in those who were active outside of recreation versus those who were less active. These results suggest that sociodemographic and behavioral variables modify the relationship between parity and body weight, and provide insight for identifying women who are at risk for having greater BMI with higher parity. This information may be applicable to the targeting and design of interventions to prevent postpartum weight retention.