The invention relates to determining a fetal heart rate from an ultrasonic Doppler echo signal. Rather than extracting the Doppler signal from the carrier signal using synchronous demodulation, quadrature demodulation is used, which allows to distinguish between systolic and diastolic movements. This velocity direction can be used to cut out unwanted parts of the signal (systolic or diastolic heart activity), thus eliminating double counting heart rates. In particular, a binary sign signal (15′) may be used to cut out unwanted signal episodes, resulting in a processed reference signal (10′) having only the signal parts of one heart activity. Another aspect relates to determining a first and a second heart rate from at least two channels having a different depth range, and selecting one of the first and second heart rate based on external information of the fetal or maternal heart rate.