Magnetic field sensor measures fetal heartbeat

To detect cardiac arrhythmias in unborn babies, their heartbeats are monitored with the aid of an electrocardiogram (ECG), among other techniques. During such monitoring, electrodes are placed on the mother’s skin. However, the baby’s electrical signals are muffled by the surrounding protective layer (vernix) during certain parts of pregnancy and can be masked by the mother’s own signals. By contrast, magnetic sensors are advantageous because they can pick up heart signals in a form which is less influenced by insulating layers. Furthermore, they do not require electrical contact with the skin.

For this reason, the German-American team of scientists tested μOPMs (microfabricated optically pumped magnetometers). Optical magnetometers are based on the notion that the electron spin of atoms will precess in magnetic fields at a precisely known frequency. In the case of Alkali atoms such as rubidium used in μOPMs, macroscopic polarization of the electron spin can be achieved in the gas phase by means of optical pumping. In this state, the atomic vapor is optically transparent. When a magnetic field is applied, the transparency is lost due to the precession of the electron spin; the measured change in absorption is a measure of the applied field strength.

The prototype which the researchers have developed is composed of three belts placed around the pregnant woman’s torso. The belts have a flexible array of 25 μOPM sensors. At a distance of approx. 4.5 mm from the skin, these sensors measure the magnetic signals generated by the heartbeats of the mother and the child without direct skin contact. The belt below the mother’s chest is used to detect her heartbeat separately. It is subtracted from the overall measurement in order to extract the signal of the child’s heart (the weaker of the two signals).

The measurements are a step towards a user-friendly and informative fetal heartbeat measurement device which can be applied at any time during pregnancy directly to the mother’s body.