A continuous, noninvasive fetal heart rate measurement is produced using one or more ultrasonic transducer adhered to the abdomen of the mother. Each ultrasound transducer generates an ultrasound beam having a signal strength. The signal strength is determined by an excitation voltage applied to the ultrasound transducer. An excitation voltage adjustment device is positioned between an excitation voltage generator and the ultrasound transducer to selectively control the strength of the ultrasound beam. A user input device allows an operator to control the ultrasound signal strength to vary the depth of viewing of the fetal heart rate monitor.
