## 3 Comparison of Fetal Heart Rate Monitoring Techniques
### Fetalelectrocardiography (FECG)  
For the fetus, fetal electrocardiography (FECG) helps to analyze FHR and health conditions because the heart of the fetus is the first organ to develop during the first 3-4 weeks of pregnancy. To determine FHR of the fetus, several fetal monitoring techniques, both invasive and non-invasive, are used. However, considering the comfort of the patient, a non-invasive process for FECG is usually preferred [67, 68, 69, 70, 71]. In [72], [PERSON] et al. analyzed and evaluated the FECG device's accuracy in the context of fetal scalp electrodes (FSE) by comparing the results from the experiment. Thirty-two pregnant women were involved for data recording and the electrode placement was achieved following the configuration shown in Figure 7. The signal quality varied according to fetal positioning. However, the results of the study depicted that an average correlation of 0.96 was achieved in comparison to results obtained using invasive fetal scalp electrodes (FSE).  
In [69], the authors used a commercially available FECG device AN24 to continuously monitor the fetus and to determine the fetus health. Some of the commercially available NIFECG devices are shown in Figure 8.  
Figure 6: An illustration of the Doppler sound procedure for FHR monitoring.  
Figure 7: Electrode placement for FECG.  
To understand the importance of electrode placement, a study was non-invasively conducted using the Monica AN24 Monitor with five electrodes [41, 69, 72, 73, 74]. It was observed that the non-invasive FECG was a preferred method to provide comfort to pregnant women in comparison to invasive FECG [75, 76, 72]. However, the results indicated that the signal quality of the FECG data obtained was affected by maternal cardiograph (2019) (2019, Creative Commons License).  
Figure 8: Examples of CE/FDA-certified commercially available NIFECG-based devices [69]. (**a**) Monica AN24, (**b**) Monica Novii Wireless Patch System, (**c**) MERIDIAN M110 Fetal Monitoring System, (**d**) PUREtrace, (**e**) Nemo Fetal Monitoring System (Reprinted with permission from [PERSON] (2019) (2019, Creative Commons License)).  
(MECG) artefacts and noise, but the FECG signal can be extracted after applying a set of filters [42, 77, 78, 79]. In [7], a study was conducted using three pairs of electrodes which were placed on the abdomen of pregnant women. Complex continuous wavelet transform (CCWT) was deployed for the extraction of the FECG signal, using a four-stage procedure, as shown in Figure 9.  
In the first stage, the signals coming from the three pairs of electrodes were averaged, followed by stage two comprising of CCWT and MECG detection. The detection and extraction of FECG was carried out in stage three, while stage four focused on analyzing the fetal heart rate (FHR). Furthermore, a method to extract FECG from the MECG using a multivariate singular spectrum analysis (MSSA) was used, comprising of two stages known as decomposition and reconstruction. This technique helped the researchers in detecting the FHR and separating the unwanted noise both in stationary and non-stationary signals. The main advantage of FECG monitoring is that it can be used remotely, in a non-clinical environment with high simplicity [7, 80].