Authors - Saki Matsudo, Yurika Obata, Koichiro Kido, Kenji Ozawa Abstract - We present First-B/M, a smartphone-based system that enables pregnant women to measure fetal heart rate (FHR) at home, analogous to auscultation. The system integrates two external microphones embedded in short stethoscope tubes, connected to an iOS device. The smartphone performs low-pass ltering (250 Hz), harmonic/percussive sound separation (HPSS) to extract fetal heart sounds, and FHR es- timation based on frame-wise amplitude increases. To improve robust- ness in non-clinical environments, a median-based temporal aggregation method is applied. A user-centered application supports recording, FHR visualization, and data sharing with clinicians. Usability was assessed with 10 participants through task completion and ve-point rating evaluations. System performance was evaluated using 20 recordings from pregnant women, in which fetal heart sounds were identiable in 12 cases. Supplementary pseudo-fetal data, generated by time-scaling adult heart sounds, were used to examine algorithm behavior under ideal con- ditions. When fetal heart sounds were captured, estimated FHR values agreed with human reference measurements within 3%. One outlier occurred under strong mid-recording noise, indicating the need for auto- matic re-measurement support. These results demonstrate the feasibility of smartphone-based auscultation for home FHR monitoring and provide a practical foundation for non-clinical FHR measurement systems.
