When you are committed to safe deliveries and healthy children, you want to be able to rely on a monitoring system that keeps track of maternal and fetal health. Maternal and fetal comfort are also important. Few reliable and comfortable technologies were available until now.

Our wireless, non-invasive, user-friendly solution is currently being used in hospitals in Germany, Austria, Belgium and the Netherlands. Over ten years of research and development preceded the development of the Nemo Fetal Monitoring System. 

Even when the CTG is obtained in a safe and reliable way, it is difficult for healthcare practitioners to interpret the CTG, there are major differences and inconsistencies between interpretations of individual practitioners [3] and the positive predictive value for a poor outcome for the foetus is low [4].

In other words, in many situations, a sub-optimal CTG does not necessarily mean that the foetus is compromised. In the event of a questionable diagnosis, midwives rely on additional methods, such as blood sampling from the foetus or analysis of ST-interval segment changes in the fetal ECG. These are both invasive methods and are associated with a risk of complications.

An alternative way to obtain information about the foetus and uterus is non-invasive electrophysiology. The fetal heart and uterus are both composed mainly of muscle tissue. Muscles contract under the influence of an electrical stimulus (action potential) that spreads across the muscle. This spreading response to electrical stimulus can be measured using surface electrodes on the skin.

Applying electrodes to the mother’s abdomen enables fetal heart and uterine muscle activity to be measured. Unfortunately, these electrodes also measure electric fields from other sources, including maternal heartbeat, abdominal muscles (especially during active pushing in the second stage of labour) and even the ever present power grid.

The image below shows a typical non-invasive electrophysiological measurement of an expectant mother. The most important problem in non-invasive electrophysiology for fetal monitoring is separating the signals. Those originating from the fetal heart and uterine muscle must be separated from all other electrical interference. In this process, electrophysiological information from the uterus is called electrohysterography and fetal electrocardiography describes the electrophysiology of the fetal heart muscle.

Electrohysterography (EHG) is a promising non-invasive technique to measure uterine electrical activity via contact electrodes on the mother’s abdomen [5]. This has a good correlation with measuring intrauterine pressure. Signal processing methods are needed to convert the EHG signal so that it resembles the intrauterine pressure curves. 

In most studies, these signal processing methods can only be used offline, which means that data first need to be collected from a patient and processed later to obtain a CTG. In other words, data are processed with a significant delay between input and output. In CTG interpretation, fetal heart rate is assessed in relation to uterine activity, which means that the EHG must be processed in real time, without significant delay.

Our professionals have developed pioneering signal processing methods for real-time processing of EHG signals, enabling these to be used for CTG interpretation. These methods are currently used in the Nemo Fetal Monitoring System.

The performance of these methods has been subject to extensive comparison with external tocodynamometry, especially in relation to maternal BMI. By comparison and using intrauterine pressure as the gold standard, these analyses show that our method had a sensitivity of 89.5% (compared with 65.3% for tocodynamometry) in a group of 48 women during labour [1].

Sensitivity in this case is defined as the number of positive contractions observed with a simultaneously used intrauterine pressure catheter. Contractions were considered as a true positive if the peak fell within 30 seconds of the peak in the intrauterine pressure signal.

In the EHG-based method, sensitivity was not affected by BMI: for the 33 non-obese women (i.e. BMI < 30 kg/m2 before pregnancy), sensitivity was 90.0% and for the 15 obese women (BMI ≥ 30 kg/m2), sensitivity was 88.4%. For external tocodynamometry, these sensitivities were 73.0% and 45.8%, respectively, with significantly worse performance for women with a high BMI [1]. Other studies have also found EHG to be superior to tocodynamometry [9-12].

The image below shows a sample measurement from the comparison study. From top to bottom, the diagrams show: fetal heart rate, intrauterine pressure, EHG, tocodynamometry. In this image, uterine contractions measured by EHG are more clearly visible than those measured by tocodynamometry. It should be noted that these diagrams are displayed at 2 cm per minute.

Fetal heart rhythm is generally measured non-invasively using Doppler ultrasound. If signal quality is low during delivery, midwives and obstetricians often have to switch to using the invasive fetal scalp electrode. This provides a good quality signal based on the fetal electrocardiogram (fECG).

A non-invasive alternative is to measure fECG by applying electrodes to the mother’s abdomen [13]. This fECG is the electrophysiological signal produced by the fetal heart during each cardiac contraction. This method enables reliable measurements of fetal heart rhythm and can also be used antepartum.

The disadvantage of the minimum invasiveness of an abdominally measured fECG is the lower signal-to-noise ratio (SNR) [14]. As indicated above, the abdominally measured fECG is contaminated by electrical interference such as the maternal ECG (MECG), muscle activity, power line interference and measurement noise. Between 27 and 36 weeks of gestation, the fetal skin is also covered by an insulating layer (the vernix caseosa) which reduces the amplitude and influences the shape of abdominally measured fECG [15].

Abdominally measured fECG recordings have been researched extensively in recent years. Most studies focused on suppressing the MECG, which is the dominant disturbance [13, 16-21]. Various algorithms have been presented for MECG suppression, including subtraction of the maternal template [13, 16, 17], adaptive filtering [18, 19], blind source separation (BSS) [20-22], or a combination of different algorithms [23-25]. Please refer to [14] or [26] for a detailed overview.

Fetal QRS complexes must be detected to determine the fetal heart rhythm. These QRS complexes reflect the electrical activity of cardiac muscles involved in ventricular contraction. However, even after MECG suppression, the SNR of the abdominally measured fECG is generally still too low for reliable fetal QRS detection. In addition to the low SNR, the position and orientation of the foetus in the uterus are unknown in advance and can change during measurement, which is why an abdominal fECG is usually performed using several electrodes spread across the abdomen [27].

The SNR and fECG waveform in each channel depend on the position and orientation of the foetus. Fetal movements relative to the abdominal electrodes may then cause variations in a particular channel’s SNR and fECG waveform [28]. In summary, low SNR and the non-stationary nature of abdominal fECG may hinder detection of fetal QRS.

The Nemo Healthcare professionals have developed patented methods to obtain a reliable fetal heart rhythm by suppressing MECG and other disturbances and detecting fetal QRS complexes, even in non-stationary conditions with low SNR. These methods were evaluated in a scientific study in several hospitals, where a fetal scalp electrode and the Nemo Fetal Monitoring System were used simultaneously in a group of 110 patients. This study (published in 2019) showed that Nemo Healthcare’s methods achieved a reliability of 86.8%, with the fetal heart rhythm of the fetal scalp electrode as a reference and an accuracy of -1.46 heartbeats per minute (BPM). Reliability here is the ratio of fetal heartbeats identified within a margin of ten beats around the fetal heart rhythm measured using the fetal scalp electrode. Literature shows that Doppler ultrasound performance is much lower, with a reliability of 62% - 73%. These studies defined reliability as the relative number of heartbeats that fall within a 10% margin of the fetal heart rhythm measured using the fetal scalp electrode. In general, this is a margin of about 14 BPM [29, 30].

The image below shows a simultaneous recording of fetal heart rate using both the Nemo Fetal Monitoring System and a fetal scalp electrode. The blue line represents the heart rhythm measured using the fetal scalp electrode, with the red line representing the heart rhythm measured using the Nemo Fetal Monitoring System.

The signal processing methods used to suppress the MECG accurately determine the maternal heart rate. This allows the Nemo Fetal Monitoring System to avoid confusion between the maternal and fetal heart rates. This problem can arise when fetal heart rhythm is measured with Doppler ultrasound based monitoring technology [31].

As noted earlier, correct interpretation of fetal health status without complementary diagnostic measures such as fetal blood sampling is not always possible. Not even in the case of a reliable CTG.

The electrophysiological measurements recorded using the Nemo Fetal Monitoring System offer ample opportunities to develop other complementary diagnostic measurements.

The Nemo Fetal Monitoring System electrode patch, for example, incorporates several electrodes. In theory, these electrodes enable measurement of conduction velocity in the EHG, which has been reported to be useful in predicting imminent delivery. This could provide opportunities to distinguish between Braxton-Hicks contractions and uterine activity that actually leads to dilation. This improves diagnosis of the risk of a pre-term birth.

The Nemo Fetal Monitoring System provides more reliable fetal heart rhythm information than Doppler ultrasound. Much current scientific research focuses on the quantitative analysis of fetal heart rate variability to predict fetal hypoxia [32-34] or intrauterine growth restriction [35].

As the fetal heart rhythm measured using the Nemo Fetal Monitoring System is based on the fECG, the fECG waveform can also be obtained and analysed. It is known that the fetal ST-interval segment changes under the influence of oxygen deprivation [36]. A non-invasive analysis of the fetal ST-interval segment or another fECG interval or segment could therefore be possible.

The multichannel fECG obtained using the Nemo Fetal Monitoring System can also be used to screen for congenital heart disease. The image below shows an example of fECG wave patterns obtained using the Nemo Fetal Monitoring System in a developmental setting. Here, each fECG was recorded using a different electrode, which explains the different pattern. These fECGs were recorded at two locations on the mother’s abdomen for the same foetus at the same time.