: This study investigates how the phenomenon of part-time work relates to the career cost of parenthood. I start by demonstrating mothers' labor supply responses to the school entry of their own and their colleagues' youngest children. Next, I assess heterogeneities across sectors with differing prevalence of part-time work. I study these relationships using Dutch administrative data on monthly employment outcomes. Adopting existing methodologies from the child penalty and peer effects literatures, I show that there are substantial differences across sectors in mothers' labor supply responses to their own child's school entry and to their colleagues' labor supply responses. Mothers in sectors with a higher prevalence of part-time work tend to increase their hours worked after school entry, but mothers in sectors with a lower level of part-time work do not. The increase is associated with a lower within-couple gender gap in hours worked for those mothers. In contrast, I find positive peer effects on hours worked that are larger in sectors where full-time work is more prevalent. These findings highlight the importance of a better understanding of the complex relationship between mothers’ return to the labor market and the prevalence of part-time work.