Estimating the causal effect of a first child on female labor supply is complicated by the endogeneity of fertility. This paper addresses this problem by focusing on a sample of women from the National Survey of Family Growth (NSFG) who sought help to become pregnant. After a certain period, only some of these women gave birth. Results using this strategy show that having a first child younger than one year old reduces female employment by 26 percentage points. These estimates are close to OLS estimates from census data and to those from OLS and fixed-effects models on NSFG data.