Objective In 2006, the incidence of unintended pregnancy among rural-to-urban migrant women (RUMW) in Shanghai was reported as 12.8 per 100 women-years during the first year postpartum. Among permanent residents of Shanghai, that same rate was 3.8 per 100 women-years. An intervention study was designed to address the unmet need for family planning services among this underserved population of RUMW and reduce their high postpartum unintended pregnancy incidence. Study design We enrolled 840 migrant women into an intervention study that provided free contraceptive counseling and a choice of methods. Subjects were recruited into the study during hospitalization for childbirth and offered a contraceptive method according to their choice prior to discharge. Counseling and further support were offered at 6 weeks and at 3, 6, 9 and 12 months postpartum via scheduled telephone calls and/or clinic visits. Results Among all study participants, the median time to contraceptive initiation and sexual resumption was 2 months postpartum, respectively. The overall contraceptive prevalence at 12 months was 97.1%, and more than half of the women were using long-acting contraception. The incidence rate of unintended pregnancy during the first year postpartum was 2.2 per 100 women-years (95% confidence interval: 1.3–3.6). Conclusions Integrating free family planning services into existing childbirth delivery services in a maternity setting in Shanghai was effective in addressing the unmet need for family planning and reduced the risk of unintended pregnancy during the first year postpartum. Implications The maternity setting at the time of early labor and prior to postpartum hospital discharge is a practical venue and an optimal time to provide contraception counseling and for postpartum women to initiate use of contraceptive methods. Supporting services during the first year postpartum are also essential to encourage women to continue contraceptive use and reduce the incidence of postpartum unintended pregnancy.