A simpler way to assess foetal health could be to use low-cost vibration sensors placed on the mother's abdomen to record acoustic signals emitted by the foetal heart. Such sensors could be operated by the mother in their own home and at any time, with the recordings analysed afterwards by the doctor. Unfortunately, however, the low acoustic energy of the foetal heartbeat makes it difficult to distinguish the foetal heart sound from background signals such as the mother's heartbeat, breathing and digestive noises.

Now, a research team from Greece and Abu Dhabi has developed a technique to accurately isolate foetal heart sounds from background noise in acoustic recordings (Front. Bioeng. Biotechnol. doi: 10.3389/fbioe.2017.00049).

"We wanted to see if a mathematical analysis technique called Wavelet Transform-Fractal Dimension [WT-FD], which has been successfully used to de-noise lung and bowel sounds, could be used to examine foetal heart sounds more accurately," explained Elisavet Koutsiana from the Aristotle University of Thessaloniki.

Koutsiana and colleagues first tested and perfected their noise-filtering technique using a database of simulated foetal heart recordings. They then examined recordings from three pregnant women, with the acoustic signal produced by the foetal hearts recorded using low-cost vibration sensors embedded in plastic harnesses.

Using the WD-FT technique to process the sounds, the researchers could clearly discern the foetal heart sounds in both foetal recordings and simulations of healthy hearts, and only slightly less clearly in simulations of abnormally fast or slow heartbeats. They could still recognize the heart sounds even when unexpected noises were present. In many cases, the team could discern the foetal heart sounds well enough to distinguish between the different segments of the foetal heartbeat - the systolic beat S1 and the diastolic beat S2 - a feature that could be useful for assessing foetal health.

"Our work suggests that there is potential for low cost and continuous recordings of foetal heart sounds in the home," said Koutsiana. "We plan to continue the research with more real recorded signals to learn more about the foetal heart cycle and how it relates to health, and also to improve the accuracy of the system further. Our results should help doctors to make pregnancy safer."