This disclosure presents a ballistocardiographic method and monitoring system for calculating an indicator of the average heart rate of a test subject within a time interval. The test subject may be a fetus inside a mothers belly. The test subject may also be an infant lying in the same bed as an adult. The monitoring system comprises a recoil surface and an acceleration sensor in contact with the recoil surface. A control unit identifies in the time domain the characteristics of the adults heartbeat during the time interval. The output signal is then transformed to the frequency domain across the time interval and the signal is screened with the adult heartbeat characteristics. The indicator is then calculated from the screened signal in the frequency domain. The control unit monitors the indicator of the infant or fetus heart rate and may provide an alert if the indicator value drops very rapidly, or falls below a predetermined threshold value.
