Unmet need for contraception is highest within 12 months post-delivery, according to research. Using longitudinal data from the Nairobi Urban Health and Demographic Surveillance System, we assess the dynamics of contraceptive use during the postpartum period among women in Nairobi’s slums. Results show that by 6 months postpartum, 83 percent of women had resumed sexual activity and 51 percent had resumed menses, yet only 49 percent had adopted a modern contraceptive method. Furthermore, 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, though the effect was much reduced after adjusting for other variables. To reduce unmet need, effective intervention programs are essential to lower high levels of discontinuation and encourage switching to more effective methods.