Using data on MZ (monozygotic, identical) female twins from the Minnesota Twin Registry, we estimate the causal effect of schooling on completed fertility, probability of being childless and age at first birth, using the within-MZ twins methodology. We find strong cross-sectional associations between schooling and the fertility outcomes and some evidence that more schooling causes women to have fewer children and delay childbearing, though not to the extent that interpreting cross-sectional associations as causal would imply. Our conclusions are robust when taking account of (1) endogenous within-twin pair schooling differences due to reverse causality and (2) measurement error in schooling. We also investigate possible mechanisms and find that the effect of women’s schooling on completed fertility is not mediated through husband’s schooling but rather through age at first marriage.