Design of an ultrasound transducer for continuous fetal heartbeat monitoring

Stillbirth is when a baby is born without signs of life after 24 weeks of gestation. Stillbirth prevention depends on healthcare access and in-time diagnosis. Moreover, antenatal tests are not always effective for stillbirth prediction and evaluation. Pregnancy home monitoring can possibly aid stillbirth diagnosis. The aim of this study is to investigate Doppler solutions for pregnancy home monitoring that are robust, reliable, easy to use and affordable. Transducers of circular flat, concave and convex shapes were investigated. Focal lengths, transducer diameters and the heart position relative to a transducer surface were varied to find a balance between the signal strength and system robustness i.e. ability to detect the fetal heartbeat if fetus and transducer are misaligned due to the fetus or transducer movements during the measurements. We have found that a circular and flat transducer of 20 mm diameter is robust up to 3.9° angle misalignment. Concave transducer with a diameter of 30 mm and 5 cm focal length receive signals up to 24.7° angle misalignment. Whereas, convex transducer of 30 mm width & 10 mm height and 100 mm radius of curvature show the maximum robustness with detection up to 32.3° angle deviation at -60 dB sound pressure level.