Women who monitor their fertility signs and recognize when they are fertile can use this knowledge to conceive or to avoid pregnancy. Studies have shown that there is a rather small fertile window of several days during each menstrual cycle. Established methods of identifying the fertile window, such as the Ovulation and the Symptothermal methods of Natural Family Planning, can be very effective in helping couples avoid pregnancy. A new algorithm for identifying the fertile window has been developed, based on monitoring and recording of cervical secretions. The TwoDay Algorithm appears to be simpler to teach, learn, and use than current natural methods. A large existing data set from a World Health Organization study of the Ovulation Method, along with Natural Family Planning charts from women using the Ovulation Method and the Symptothermal Method, were used to determine the potential effectiveness of the TwoDay Algorithm in identifying the fertile window. Results suggest that the algorithm can be an effective alternative for low literacy populations or for programs that find current Natural Family Planning methods too time consuming or otherwise not feasible to incorporate into their services. Further studies are needed to determine the efficacy of the TwoDay Algorithm in avoiding pregnancy and to assess its acceptability to users and providers.