Families formed through multipartner fertility, where children with a common biological mother were conceived by different biological fathers, represent a growing share of all families in the United States. Using data from four waves of the Fragile Families Child and Well-Being Study (N=3,366), I find that women who have engaged in multipartner fertility are more likely to experience parenting stress and depression compared to mothers whose children share the same biological father. Mothers' depression is explained in the short term by poor relationship quality with the father of her prior children and in the longer term by indicators of boundary ambiguity in complex families. Mothers' parenting stress was only weakly explained by variation in perceived kin support, father involvement, or boundary ambiguity.