This paper documents the evolving impact of childbearing on the work activity of mothers. Based on a compiled dataset of 441 censuses and surveys between 1787 and 2015, representing 103 countries and 48.4 million mothers, we document three main findings: (1) the effect of fertility on labor supply is small and typically indistinguishable from zero at low levels of development and economically large and negative at higher levels of development; (2) this negative gradient is remarkably consistent across histories of currently developed countries and contemporary cross-sections of countries; and (3) the results are strikingly robust to identification strategies, model specification, data construction, and rescaling. We explain our results within a standard labor-leisure model and attribute the negative labor supply gradient to changes in the sectoral and occupational structure of female jobs as countries develop.