This study examines whether a birth of higher parity affects mothers' well-being. Literature documents that well-being is linked to a subsequent birth but differs in its conclusions about how and why. In particular, the underlying processes are not yet understood. There is some evidence that a subsequent birth impairs coparenting, the way parents work together, because parents have to adapt to and reorganize this new-person-in-context constellation. Also, the financial costs of children and the pressure this might put on the family system have to be considered. Applying the framework of the Family Stress Model, we examine the process of how a subsequent birth affects mothers' well-being by considering both factors as mediators in a structural equation model. Using German two-wave panel data (AID:A 2009 and 2014; German Youth Institute, 2019) on 3,738 nuclear families, our findings suggest that, after a subsequent birth, if mothers perceive a deterioration in coparenting and their personal income increases at a lower rate, then these factors mediate the association between a higher parity parenthood and decrease mothers' well-being. The findings are important for prevention and intervention because they suggest that a subsequent birth can negatively affect familial resources, such as the quality of coparenting, with risks for partnership stability as well as child development. The findings are also important for family policy because they indicate that a worsened personal financial situation after a subsequent birth affects mothers' individual well-being negatively and might thus be relevant for further fertility intentions. (PsycInfo Database Record (c) 2021 APA, all rights reserved).