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. So that the method can be carried out by a woman herself quickly and nevertheless with high precision, it is provided according to the invention that the device (1) is embodied as a hand-held device and an algorithm is stored in the processor (7), with which with a measurement based on several immediately consecutive breaths a predetermined reproducibility criterion regarding an end expiratory CO2 partial pressure of the respiratory gas can be tested, wherein, if the reproducibility criterion is met, the at least one output unit (9) outputs and/or signals and/or displays whether a fertile phase applies. Furthermore, the invention relates to a method for determining a fertile phase of a woman by ascertaining an end expiratory CO2 partial pressure in a respiratory gas and comparison of the measured data thus obtained with measured data in the follicle phase outside the fertile phase.
