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Magnetic field sensor measures fetal heartbeat

Contactless heartbeat monitoring during pregnancy

PTB-News 2.2016
Especially interesting for


prenatal diagnostics

PTB researchers have measured the magnetic signals generated by the heart activity of the unborn child of a pregnant woman by using optical magnetic field sensors on her abdomen. They have demonstrated that the technology developed by the US National Institute of Standards and Technology (NIST) measures as reliably as other methods, and that it can even provide additional information. This might permit fetal heart defects to be diagnosed at an early stage.

The measured field distribution of the maternal heart (left) and the fetal heart (right). The red iso-field lines denote the areas with a positive sign of the magnetic field; the blue lines denote those with a negative sign. The position of each heart in this diagram is only approximate, as the exact positions were not determined precisely in this test measurement. Both heartbeat signals were measured simultaneously and then separated by means of a mathematical algorithm.

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.


Tilmann Sander-Thömmes
Department 8.2 Biosignals
+49 (0)30 3481-7436

Scientific publication

O. Alem, T. H. Sander, R. Mhaskar, J. LeBlanc, H. Eswaran, U. Steinhoff, Y. Okada, J. Kitching, L. Trahms, S. Knappe: Fetal magnetocardiography measurements with an array of microfabricated optically pumped magnetometers. Phys. Med. Biol. 60, 4797–4811 (2015)