Abstract: In this paper we introduce a new IV strategy based on IVF induced fertility variation in infertile families to estimate the causal effect of children on their parents' labor supply using the census of IVF treated women(and spouses) in Denmark. Because the observed chances of IVF success do not depend on the labor market histories of women before they enter the IVF treatment, IVF treatment success provides a plausible instrument for childbearing among childless women. IV estimates show that women work significantly less because of childbearing. The labor supply consequences are the largest during the first two years of the child's life and are still observed when children are about ten years old.