Healthy Pregnancy, Healthy Baby (HPHB) is a mobile health text-messaging service available across four major mobile network operators (MNOs) in Tanzania – Airtel, Tigo, Vodacom and Zantel. The service delivers localised, timely and tailored nutrition and health content to pregnant women, mothers and their supporters. As of November 2017, over 1.8 million users have cumulatively received over 115 million messages making HPHB one of the largest mHealth deployments in Sub-Saharan Africa. In addition to its impressive scale, the service has had significant impact on health outcomes among end-users with 73 per cent of HPHB users implementing appropriate breastfeeding practices in their households compared to only 64 per cent of non-users. From 2014 onwards, the GSMA mHealth programme provided service design and consultancy support to identify opportunities to enhance the service. The programme also provided in-kind monitoring and evaluation (M&E) support to investigate how mobile can drive positive health outcomes.