An electronic system for determining a fertility phase of a female human comprises a wearable device (1) that includes a first sensor system (101), configured to determine heart rate and heart rate variability of the female human, and a second sensor system (102), configured to determine acceleration of the female human. The electronic system further comprises a processor (13, 30), configured to detect during menstrual cycles sleep phases with resting pulse, using the heart rate variability and the acceleration, to determine a change in the resting pulse during a menstrual cycle using the heart rate, and to determine the fertility phase using a time of the change in the resting pulse. The processor for determining the fertility phase is implemented in the wearable device (1), in a mobile communication device (4), and/or in a cloud-based computer system (3).
