This paper outlines a method of reducing the data rate for transmitting fetal Doppler ultrasound audio signals. A specific application is cited where compression of the fetal Doppler signal is required to transmit information to cardiotocographs (CTGs) using radio telemetry. The method involves splitting the signal into amplitude and frequency components. The amplitude is represented by samples of the signal envelope whilst the frequency information is represented by the number of zero-crossings within fixed intervals (windows). With a careful choice of window size, it is shown that this method can be used to reproduce the signal with no audible difference when compared with the original waveform. A reduction in data rate of 15:1 is achieved.