DopFone: Doppler-Based Fetal Heart Rate Estimation Using Commodity Smartphones

Fetal heart rate (FHR) monitoring is critical for prenatal care, yet current methods rely on expensive, specialized equipment and trained personnel, limiting accessibility in low-resource and at-home settings. We present DopFone, a novel approach leveraging the built-in speaker and microphone of commodity smartphones to non-invasively estimate FHR via Doppler-based audio sensing. Our system emits an 18 kHz low-pitched ultrasound from the smartphone speaker and analyzes reflected signals recorded by the microphone to detect abdominal surface vibrations caused by fetal cardiac activity. Combining Doppler sensing with an AdaBoost regression model (validated via Leave-One-Out Cross-Validation on 23 pregnant participants), DopFone achieved a mean absolute error of 2.1±1.3 BPM compared to a reference-standard medical Doppler device. The 95% limits of agreement (±4.90 BPM) fall well within the clinically acceptable threshold of ±8 BPM. The system demonstrated robustness across gestational ages (19-39 weeks), maternal BMI (23-67 kg/m 2 ), and variations in phone positioning. Our results establish that smartphones can deliver clinically reliable FHR estimation without external hardware, gel, or probes, bridging the gap between clinical monitoring and accessible at-home assessment.