BACKGROUND: Ensuring universal access to maternal and reproductive health services is critical to the success of global efforts to reduce poverty and inequality. Engaging private providers has been proposed as a strategy for increasing access to healthcare in low- and middle-income countries; however, little consensus exists on how to estimate the extent of private sector use. Using research from sub-Saharan Africa, this study systematically compares and critiques quantitative measures of private sector family planning and childbirth service use and synthesizes evidence on the role of the private sector in the region. METHODS: We conducted a systematic review of the Medline, Global Health, and Popline databases. All studies that estimated use of private sector of family planning or childbirth services in one or more sub-Saharan African countries were included in this review. For each study, we extracted data on the key study outcomes and information on the methods used to estimate private sector use. RESULTS: Fifty-three papers met our inclusion criteria; 31 provided outcomes on family planning, and 26 provided childbirth service outcomes. We found substantial methodological variation between studies; for instance, while some reported on service use from any private sector source, others distinguished private sector providers either by their profit orientation or position within or outside the formal medical sector. Additionally, studies measured the use of private sector services differently, with some estimating the proportion of need met by the private sector and others examining the sector's share among the market of service users. Overall, the estimates suggest that the private sector makes up a considerable portion (> 20%) of the market for family planning and childbirth care, but its role in meeting women's need for these services is fairly low (< 10%). CONCLUSIONS: Many studies have examined the extent of private sector family planning and childbirth service provision; however, inconsistent methodologies make it difficult to compare results across studies and contexts. Policymakers should consider the implications of both private market share and coverage estimates, and be cautious in interpreting data on the scale of private sector health service provision without a clear understanding of the methodology.