We extended Nomaguchi and Milkie’s (2003) analysis of changes in well-being (as measured by changes in social and psychological resources) associated with the birth of a first child. We used the same data source, the National Survey of Families and Households (NSFH), and a similar methodological approach. A key difference between our analysis and the prior work is that we focused on how the effect of a nonmarital birth varies by cohabitation status. The results of this extension suggest that having a child is similarly associated with well-being for married and single women, but cohabiting women experience remarkable declines in social and psychological well-being, even controlling for their (often lower) level of well-being prior to the birth of the child. That is, cohabitation is associated with particularly problematic mental health outcomes for new mothers.