Generally, the FHR is obtained non-invasively using Doppler ultrasound. In the event of poor signal quality during labour, obstetricians often switch to the invasive fetal scalp electrode, which provides a good quality signal based on the fetal electrocardiogram (FECG).
A non-invasive alternative is to measure the FECG non-invasively using electrodes placed on the maternal abdomen . This FECG is the electrophysiological signal generated by the fetal heart during each cardiac contraction. This method has the potential to provide reliable FHR measurements and can also be applied antepartum.
The low invasiveness of the abdominal FECG comes at the cost of a reduced signal-to-noise ratio (SNR) . As mentioned above, the abdominal FECG is contaminated by electrical interferences such as the maternal ECG (MECG), muscle activity, power line interference, and measurement noise. Moreover, in the period between 28 to 32 weeks of gestation, an insulating layer (the vernix caseosa) coats the skin of the fetus and reduces the amplitude and affects the shape of the abdominal FECG .
In recent years, abdominal FECG recordings have been extensively studied, most studies focussing on suppression of the MECG, which is the dominant source of interference [13, 16-21]. A variety of algorithms has been presented for MECG suppression, such as template subtraction [13, 16, 17] adaptive filtering [18, 19], blind source separation (BSS) [20-22], or a combination of different algorithms [23-25]. For an extensive review, the reader is referred to  or .
To determine the FHR, one needs to detect the fetal QRS complexes. These QRS complexes represent the electrical activity of the cardiac muscles involved with contraction of the ventricles. However, even after MECG suppression, the SNR of the abdominal FECG is generally still too low for reliable fetal QRS detection. In addition to the low SNR, the position and orientation of the fetus within the abdomen are a priori unknown and can change during a recording. Therefore, the abdominal FECG is typically recorded using multiple electrodes spread across the abdomen . The SNR and waveform of the FECG in each channel depend on the fetal position and orientation. Hence, fetal movement with respect to the abdominal electrodes can cause variations in the SNR and FECG waveform of a certain channel . In short, the low SNR and the non-stationary nature of the abdominal FECG make fetal QRS detection challenging.
At Nemo Healthcare, we have developed proprietary methods for suppressing MECG and other interferences and detecting fetal QRS complexes to yield reliable FHR, even in challenging low-SNR and non-stationary conditions. These methods have been evaluated in a multi-centre study where the Nemo Fetal Monitoring System was used in a group of 110 patients, simultaneously with a fetal scalp electrode. This study (publication expected in 2019) showed that the Nemo Healthcare methods obtained a reliability of 86.8%, with the FHR from fetal scalp electrode as reference and an accuracy of -1.46 beats per minute (BPM). Here, reliability is the ratio of fetal heartbeats identified that fall within a 10% margin around the FHR obtained from the fetal scalp electrode. From the literature, it is known that the performance for Doppler ultrasound is much lower, with a reliability of 62-73%, where reliability is again defined as the relative number of heart beats that fall within a 10% margin around the FHR obtained from the fetal scalp electrode (which typically is a margin of about 14 BPM) [29, 30].
The figure below depicts a simultaneous registration of FHR from the Nemo Fetal Monitoring System and a fetal scalp electrode. Here, the blue line represents the FHR from the fetal scalp electrode and the red line the FHR from the Nemo Fetal Monitoring System.
Finally, because the signal processing methods that are used to suppress the MECG also determine maternal heart rate (MHR), not only can the MHR be displayed by the Nemo Fetal Monitoring System, the system can also prevent mix-up issues between the MHR and FHR, which can occur with Doppler Ultrasound-based FHR monitoring technology .