Postpartum months provide a challenging period for poor women. This study examined patterns of menstrual resumption, sexual behaviors and contraceptive use among urban poor postpartum women. Women were eligible for this study if they had a birth after the period September 2006 and were residents of two Nairobi slums of Korogocho and Viwandani. The two communities are under continuous demographic surveillance. A monthly calendar type questionnaire was administered retrospectively to cover the period since birth to the interview date and data on sexual behavior, menstrual resumption, breastfeeding patterns, and contraception were collected. The results show that sexual resumption occurs earlier than menses and postpartum contraceptive use. Out of all postpartum months where women were exposed to the risk of another pregnancy, about 28% were months where no contraceptive method was used. Menstrual resumption acts as a trigger for initiating contraceptive use with a peak of contraceptive initiation occurring shortly after the first month when menses are reported. There was no variation in contraceptive method choice between women who initiate use before and after menstrual resumption. Overall, poor postpartum women in marginalized areas such as slums experience an appreciable risk of unintended pregnancy. Postnatal visits and other subsequent health system contacts provide opportunities for reaching postpartum women with a need for family planning services.