This study estimates the heterogeneous effects of the first childbirth on women's annual income in the United States using the National Longitudinal Survey of Youth data from 1979 to 2018. Women usually experience an immediate decrease in their income after childbirth relative to what they would have earned if they had not become mothers. The gap closes somewhat over time though mothers never fully catch up to their counterfactuals. Previous work tried to explain the motherhood penalty by estimating the average treatment effect of children on women's income; however, these effects can be quite heterogeneous across mothers with different observable characteristics. In this study, our interest centers on the distribution of the individual-level effect of the first childbirth on mothers' income using the Changes-in-Changes model and quantile regression. Identifying the features of this distribution is a challenging task as it requires the knowledge of joint distribution. We find that around 73% of mothers have lower income after their first childbirth relative to what they would have had if they had not had a child.. Moreover, the adverse effects of motherhood on income are particularly pronounced among 10-20% of mothers. Our quantile regression analysis, documented in the Appendix, indicates that the first childbirth most negatively affects older, single/divorced, white, more educated mothers, and those with higher potential earnings had they not given birth.