Image: Quartz. Bwenzi Lathu, a USSD app used by African women to track their fertility windows for family planning.
Unstructured Supplementary Service Data (USSD) is a protocol of the GSM standard form 1997, mostly used to allow simple queries to be sent and received to and from the mobile network, like requesting your account balance. The advent of mobile internet and smartphones have largely decimated the use of USSD in developed markets. However in Africa, where there is only 18% internet penetration, there is over 80% penetration of GSM mobile phones. As a result, developers have started using USSD as a widespread delivery mechanism for apps from banking to fertility tracking. Here is an interesting article in Quartz about the use USSD in Africa.