Nigeria is a key target country in the global effort toward elimination of mother-to-child transmission of HIV. Low coverage of prevention of mother-to-child transmission (PMTCT) interventions, adherence, and retention-in-care rates in HIV-positive pregnant women are contributing factors to high mother-to-child transmission of HIV (MTCT) rates. In Nigeria, rural areas, served largely by primary health care facilities, have particularly poor indicators of PMTCT coverage. Mentor Mothers are HIV-positive women who serve as peer counselors for PMTCT clients, provide guidance, and support in keeping appointments and promoting antiretroviral adherence and retention-in-care. The Mother Mentor (MoMent) study aims to investigate the impact of structured Mentor Mother programs on PMTCT outcomes in rural Nigeria.