Passive Smartphone-Microphone Phonocardiography for Joint Maternal-Fetal Cardiac Well-Being Monitoring: A Longitudinal Self-Recorded Case Study in Late Pregnancy

Maternal and perinatal deaths are concentrated in low- and middle-income countries, where routine fetal monitoring is largely inaccessible. With the prevalence of smartphone as personal assistive gadget, we ask whether a phone's built-in microphone, with no extra hardware or emitted signal, can serve as a software-only maternal-fetal cardiac screening aid when moved from the chest to the lower abdomen? We report a longitudinal, self-recorded case study of a single subject (Z) over the last five weeks of a singleton pregnancy, later delivered at term without complications: 28 short, uncontrolled recordings (rest, morning, post-exercise). Lacking maternal/fetal ground truth, we exploit the prior that maternal and fetal heart rates occupy largely disjoint frequency bands, blindly decomposing a Shannon-energy envelope into two candidate-rate bands, and additionally probe a slower breathing-rate band as a rest/exercise discriminator. A fast, regular, fetal-plausible rhythm was recovered from most recordings and remained essentially unchanged across maternal activity states, consistent with normal fetal cardiac autonomy; a slower, noisier maternal-plausible rhythm was recovered less reliably. Beat-to-beat variability proved largely noise-dominated at our short window lengths, and the breathing-rate probe did not significantly separate rest from exercise; both are reported as negative findings. We position the work within AI for global health: a low-cost screening aid for remote and underserved communities, pending reference-validated follow-up, not a replacement for clinical monitoring.