Growing restrictions on abortion in the U.S. and abroad make understanding why undesired pregnancies occur and the role of policy in limiting them increasingly urgent. Consensus on these questions has eluded social science for decades, largely due to data limitations and the dearth of internally valid research designs. Using a randomized control trial, we find that making all contraception free for uninsured women seeking care increases the efficacy of their chosen contraceptives and decreases the likelihood of undesired pregnancy. Scaling this policy nationally would reduce undesired pregnancies by 5.9%, abortions by 9.2%, and save 1ドル.52 billion in public expenditures in the first year. Results inform a long debate regarding the determinants of contraceptive use and effects of reproductive health policy in developed countries.