In the United States, multipartnered fertility (MPF) has become commonplace. This study provides the first nationally representative measures of women's MPF, across multiple years, using the U.S. Census Bureau's Surveys of Income and Program Participation. Because welfare rules contain strong incentives for MPF, and because MPF is especially common among welfare recipients, the authors also examine the relationship between welfare and MPF. Focusing on the pre-TANF period 1985 to 1996, when welfare rules were more comparable across states and the absence of time limits made the incentives for MPF larger, the authors find little behavioral response. Among low-income mothers, MPF does not appear to be driven by program design. Because the incentives were relatively large and reached well up the income distribution, the findings amplify those of earlier studies that show little demographic response to antipoverty programs and invite reconsideration of how much these incentives should constrain transfer programs that target children. (Contains 18 notes, 7 tables, and 5 figures.)