A transducer for recording fetal movements and sounds based on an inductive principle

Recording fetal breathing movements requires a transducer with a large dynamic range. The compliance of the transducer should be matched to the compliance of the maternal abdominal wall in order to pass the very weak acoustic signal caused by heart sounds and fetal breathing movements on to the transducer. The output signal is stored on the hard disc of a personal computer (PC) simultaneously with a control signal indicating the presence of breathing movements. Ultrasound imaging is exclusively used for verification of breathing movements. The images are stored on videotape simultaneously with control signals from the PC for synchronisation of data and image. Substantial digital filtering is necessary to discriminate between actual breathing movements and other fetal activities. It is shown that by optimal adjustment of the transducer's compliance to that of the maternal abdominal wall individual breathing movements can indeed be measured.