Unmet need for contraception is highest within 12 months following childbirth. Using data from the Nairobi Urban Health and Demographic Surveillance System, we assess postpartum contraceptive use among women in Nairobi’s slums. By 6 months postpartum, 83 percent of women had resumed sex and 51 percent had resumed menses, yet only 49 percent had adopted a modern contraceptive method. Further, almost half of women discontinued a modern method within 12 months of initiating use, with many likely to switch to another short-term method with high method-related dissatisfaction. Women who adopted a method after resumption of menses had higher discontinuation rates. Effective programs are essential to lower high levels of discontinuation and encourage switching to more effective methods.