Married women’s labor supply in the United States has increased only slowly since the late 1990s, while married men’s labor supply has continued to decline. This slowdown in married women’s labor supply in recent decades masks substantial heterogeneity across households. From the late 1990s to the early 2020s, married mothers (with at least one child under age 15) experienced large increases in work hours, whereas women without children (non-mothers) saw only small increases. Over the same period, married fathers reduced work hours more than their counterparts without children while increasing time devoted to childcare. To explain these changes, we develop a two-earner life-cycle model in which labor supply and fertility are endogenous and spouses’ childcare time is substitutable. Counterfactuals show that the narrowing gender wage gap is the primary driver of recent shifts: it raises mothers’ work hours and lowers fathers’ work hours, in part by reallocating childcare toward fathers and thereby expanding mothers’ capacity to work and accelerating their human capital accumulation.