A transmit amplifier (12) drives an ultrasound transducer (11) to emit a pulse, and a receive amplifier (13) amplifies the echo signal detected by the transducer. The receive amplifier's gate opens a fixed delay after the end of the transmit pulse, and a demodulator (14) multiplies the received signal by the local oscillator signal. A low-pass filter removes the sum of the frequencies and passes the difference of the frequencies (the received signal's Doppler frequency) to be digitized by an ADC. ADC readings are made for each of several range bins in intervals during the receive gate-open interval. One or two of the Doppler audio signals will contain the signal from the fetal heart. When a periodic signal is found, its rate is tested to see if it lies within or outside the typical range of a fetal heart.
