BACKGROUND: The use of demand-side financing mechanisms to increase health service utilisation among target groups and enhance service quality is gaining momentum in many low- and middle-income countries. However, there is limited evidence on the effects of such schemes on equity, financial protection, quality of care, and cost-effectiveness. A scheme providing free health insurance cards to poor pregnant women and their households was first introduced in two regions of Tanzania in 2011 and gradually expanded in 2012. METHODS: A controlled before and after study will examine in one district the effect of the scheme on utilization, quality, and cost of healthcare services accessed by poor pregnant women and their households in Tanzania. Data will be collected 4 months before implementation of the scheme and 17 months after the start of implementation from a survey of 24 health facilities, 288 patients exiting consultations and 1500 households of women who delivered in the previous year in one intervention district (Mbarali). 288 observations of provider-client interactions will also be carried out. The same data will be collected from a comparison district in a nearby region. A process evaluation will ascertain how the scheme is implemented in practice and the level of implementation fidelity and potential moderators. The process evaluation will draw from impact evaluation data and from three rounds of data collection at the national, regional, district, facility and community levels. An economic evaluation will measure the cost-effectiveness of the scheme relative to current practice from a societal perspective. DISCUSSION: This evaluation will generate evidence on the impact and cost-effectiveness of targeted health insurance for pregnant women in a low income setting, as well as building a better understanding of the implementation process and challenges for programs of this nature.