Abstract: A method for determining for a female a time interval for using contraception is disclosed, the method comprising receiving in a processor (11), from a sensor system (22) of a wearable device (2) of the female, physiological data of the female, generating cycle phase probabilities of the female being in one or more cycle phases of her menstrual cycle on a given day, by use of a machine learning model and the physiological data, determining the time interval for using contraception using the physiological data, the cycle phase probabilities, and pre-determined cycle phase probability thresholds, and generating a message for the female comprising the time interval for using contraception.