Abstract: The invention relates to a device (1) for determining a fertile phase of a woman by ascertaining a CO2 partial pressure in a respiratory gas of the woman in several consecutive breaths, comprising an inlet for receiving the respiratory gas, and an outlet, a sample chamber (2), into which the respiratory gas can be conducted, a measuring module (3) with a measuring element (4) with which a CO2 concentration in the respiratory gas in the sample chamber (2) can be measured, optionally a pressure sensor (5) for measuring an air pressure, a processor (7) for processing measured data obtained by means of the measuring module (3) and the pressure sensor (5), and at least one output unit (9) for outputting a result of the measured data.