A mechanism for estimating windows for menstrual cycles and fertility. The mechanism tracks menstrual cycles based on accuracy of predictions as compared to user input logging the start and stop of a cycle; this user input and the accuracy of prior cycle predictions is used to improve and update future predictions. Thus, as the mechanism receives additional data, it refines its predicted fertility window and period. A user's heart rate may be used to estimate fertility windows, periods, and other portions of a menstrual cycle.