Natural Cycles’ subscription product relies on an algorithmic method to predict which days women are fertile during their monthly cycle — with an in-app color-coded calendar informing users if they could have unprotected sex on a given day or not.

Women using the app are required to measure and input their body temperature daily using a basal thermometer. Natural Cycles relies on this and other individual data inputs to algorithmically calculate the probability of the user being fertile on each day in her cycle.