Abstract

Introduction

Fetal heart rate (fHR) observation is important for proper fetal well-being assessment during the period of pregnancy. Electronic fetal monitoring (EFM) is a significant tool for the obstetrician, in order to perform various tests at different stages of gestation to estimate the fetal health. The typical examination until the 28th week of pregnancy is composed of continuous measurements of the fetus growth, while at the stage of 29–40weeks, the monitoring of fetal movement, fetal respiration, fHR, and others (Adithya et al., 2017) are included. In current practice, the examination of the fetus is performed by means of ultrasonic-based equipment such as Doppler ultrasound and cardiotocogram (Nassit and Berbia, 2015).

Although Doppler ultrasound and cardiotocogram are the typical fetal observation devices, these techniques present some limitations, mainly because of the cost of the monitoring devices and the complexity of their use, demanding an expert during data acquisition. Moreover, it has not been established that the frequent and long-term exposure to ultrasound energy has no effect on either the fetus or the mother (Salvesen, 2002).

Existing standards of fetal monitoring estimate the fetus and the mother physiology with repetitive examinations. However, complications may occur during pregnancy, i.e., fetal deaths, preterm delivery, hypoxia, and other, which have no specific prevision. Even though the literature is not robust about the risks and their relation to the EFM, long-term fHR monitoring has proven to be an effective approach for better accuracy in the clinical examination of the fetus (Martin, 1998).

A passive alternative for long-term monitoring of the fetus is the fetal heart auscultation. It is a non-invasive method that records the vibroacoustic signals from the abdominal surface. The acoustic signal produced by the fetal heart sound (fHS) can be visually depicted in the fetal phonocardiograph (fPCG). We can separate the fetal heartbeat into two sub-beats, the systolic beat S1 and the diastolic beat S2, which follows S1. The S1 and the S2 sub-beats are generated by the vibratory components of the fetal heart valves closure. The S2 sub-beats present smoother morphology than the S1, making harder the detection of their location. A heart cycle consists of the S1 and S2 sub-beats.

The research of fPCG signals aims to segment the S1 and S2 sub-beats, in order to study the wavelet morphology of the fHSs and the fHR variability. Long-term monitoring of the fHSs reveals information about the fetus growth and functionality. Although there is not enough knowledge about fHS morphology, in order to indicate any pathological conditions, the study of the fPCG signals have shown promising results to the extension of the EFM and the physical examination of the fetus (Adithya et al., 2017).

Auscultation is a low-cost and non-invasive method as it captures the acoustic signal of the fHSs. Moreover, the phonocardiogram device is a flexible method that does not need an expert to record the signals. The mother can take long-term recordings during the day or night and afterward, the doctor can examine the signals and have a more complete overview of the fetus functionality.

Nevertheless, fetal auscultation has many challenges. Because of the place of the fetus in the maternal abdominal, the fPCG signals are loaded with noise from various sources such as maternal heart sounds, digestive sounds, maternal and fetus respiration movements, external noise, and others (Várady et al., 2003; Cesarelli et al., 2012). In the noisy fPCG signals, the fetal heartbeats are often masked by other components, consequently it is difficult to detect without applying robust signal processing methods.

Throughout the years, various signal processing approaches for de-noising the fPCG signal have been examined and proposed (Unser and Aldroubi, 1996; Messer et al., 2001; Várady et al., 2003; Xiu-Min and Gui-Tao, 2009; Chourasia and Mittra, 2010; Chourasia et al., 2011, 2014). Among them, Khadra et al. (1991) were the first to suggest the wavelet transform (WT) as a useful tool for the analysis of heart sounds. Following, many researchers concentrated on the study of wavelet-based techniques for these signals. Vaisman et al. (2012) proposed the WT as a de-noising tool for the determination of the fHR. At the same time, Kovács et al. (2011) used autocorrelation technique, WT, and matching pursuit for the evaluation of fHS. Recently, Chourasia and Tiwari (2013) designed a new wavelet basis function for de-noising the fPCG signals.

The present study was motivated from a previously proposed method of Hadjileontiadis for the separation of lung and bowel sounds from the background noise (Hadjileontiadis, 2005). The latter technique uses a scheme of WT for de-noising the signals and also fractal dimension (FD) analysis for the detection of lung and bowel sounds. The so-called WT–FD filter introduces an alternative way to the enhancement of bioacoustic signals, applicable to any separation problem involving non-stationary transient signals mixed with uncorrelated stationary background noise (Hadjileontiadis, 2005).

In this study, the WT–FD method is suggested for the case of fPCG signals, to effectively locate and extract the fetal heartbeat from the underlying noise. Due to the highly noisy environment and the low acoustic energy of the fetal heartbeat, WT is an efficient method that decomposes the signal into multiple levels for the subtraction of the unwanted stationary noise. Moreover, the method is flexible since it uses short windows at high frequencies and long windows at low frequencies making the wavelet function more similar to the waveforms of the signal. Furthermore, FD analysis is frequently used in biomedical signal processing. There are studies of FD performance at electroencephalograms for the detection of the onset of epileptic seizures and also at electrocardiogram signals for the classification of arrhythmia with satisfying results (Mishra and Raghav, 2010; Polychronaki et al., 2010).

The rest of the paper is formed as follows. Section “Mathematical Background” describes the mathematical background of WT and FD definitions, while Section “The WT–FD Method” presents the proposed method. Section “Implementation and Evaluation Issues” describes the databases that the method was tested and the general indices that used for its evaluation. Finally, Section “Results” confers some experimental results, which evaluate the efficiency of WT-FD algorithm in fPCG signals, and Section “Concluding Remarks” concludes the paper with suggestions for future work.

Mathematical Background

The WT–FD Method

WT Coefficient Estimation and Selection

In this study, the Daubechies 4 wavelet family (Daubechies, 1988) has been chosen for de-noising the signal. As described in Section “Wavelet Transform,” WT decomposes the input fPCG signal X[n] (n =1,…,N) into R detail coefficients WT_m[n] (m =1,…,R). The number R of the adjustment resolution scales is estimated by log₂ N. An example of an fPCG signal is presented in Figures ​Figures1A–H,1A–H, where the original signal is decomposed into seven levels. It is clearly depicted that the first WT level contains only noise and the last three do not contain any important components of the signal. Hence, from the R estimated coefficients the algorithm selects those including important information and leaves out those including background noise, as described next.

Open in a separate window

Figure 1

Wavelet transform (WT) decomposition on a simulated fetal phonocardiograph signal. (A) The original signal X[n]. (B–H) The seven decomposed levels of the input signal.

First, from the R estimated WT resolution levels, the first D ones are discarded according to the following criterion:

$$D=\text{min}\left\{\mathrm{\lambda }:{\mathrm{\eta }}_1^{\prime }-{\mathrm{\eta }}_{\mathrm{\lambda }+1}^{\prime }\le 0.4\right\},$$

(9)

and from the J =(R−D) coefficients, the first M ones are selected according to the following criterion (Hadjileontiadis, 2007):

$$M=\text{min}\left\{\mathrm{\lambda }:\left|{\mathrm{\eta }}_{\mathrm{\lambda }}^{\prime }\right|>p^{\wedge }\left|{{\mathrm{\eta }}^{\prime }}_{\mathrm{\lambda }+1}\right|\le p^{\wedge }{\mathrm{\eta }}_{\mathrm{\lambda }}^{″}>0\right\},$$

(10)

with

$${\mathrm{\eta }}_{\mathrm{\lambda }}=1-\frac{{\sum }_{i=1}^{\mathrm{\lambda }}E\left\{\text{W}{\text{T}}_i{\left(n\right)}^2\right\}}{{\sum }_{i=1}^{J}E\left\{\text{W}{\text{T}}_i{\left(n\right)}^2\right\}},\mathrm{\lambda }=1,2,\dots ,Jn=1,2,\dots ,N,$$

(11)

where $\mathrm{\eta }{\prime }_{\mathrm{\lambda }}$ and $\mathrm{\eta }{″}_{\mathrm{\lambda }}$ denote, respectively, the first and second derivatives of η_λ with respect to λ, p is a small number close to 0 that serves as a threshold, which accounts for the fluctuation of the first derivative around 0, and E{} denotes the expected value; here, p was empirically set equal to 0.01.

FD-Based S1 and S2 Selection

The fHS segmentation is performing using the FD method across the selected WT_j[n] (j =1,…,M) WT level. Specifically, the windowing Katz definition of FD as it is described in Section “Fractal Dimension” is performed at every selected coefficient. Then, the estimated ${\text{FD}}_j^i\left[n\right]\left(i\text{-th iteration},j\text{-th selected coefficient}\right)$ are fed to the FD-peak peeling algorithm (FD-PPA), as it is proposed by Hadjileontiadis (2005), in order to automatically detect the ${\text{FD}}_j^i$ peaks. Through a self-adjusted iterative procedure, the FD-PPA iteratively “peels” the estimated FD signal, gradually gathering those parts that construct its peaks, resulting in the ${\text{FDPP}}_j^i\left[n\right]$ sequence as it is shown in Figures ​Figures2A–C.2A–C. Hence, the algorithm aims to search for the lower peaks, such as the S2 fetus heartbeats, which correspond to the low amplitude coefficients.

Open in a separate window

Figure 2

A working example of the production procedure of the binary thresholds ${\text{SBTH}}_3^1$ and ${\text{NBTH}}_3^1$ derived from the application of the wavelet transform (WT)–fractal dimension (FD) filter to a case of fetal phonocardiograph recording. These results refer to scale j=3 and iteration k=1 during the application of the WT–FD filter to the input signal. (A) WT₃, the third WT coefficient, (B) ${\text{FD}}_3^1$, the estimated FD using Katz’s definition by Eq. 2, (C) ${\text{FDPP}}_3^1$, the output of the FD-peak peeling algorithm, (D) ${\text{SBTH}}_3^1$, the signal binary threshold, and (E) ${\text{NBTH}}_3^1$, the noise binary threshold.

In the present study, each WT_j[n] (j =1,…,M) coefficient is separated in smaller epochs for better FD assessment. Therefore, the FD estimation is more accurate considering the lower peaks. The normal duration of a fetal heart cycle is 430ms and, consequently, a mean value for each epoch is at 430ms in order to contain at least one S1 and one S2 heartbeat. Every epoch is fed in the FD-PPA iteration procedure and then reunited in the WT_j[n] (j =1,…,M) coefficient estimation.

The FD-PPA iteration procedure starts with a threshold operation based on the SD of the vector ${\text{FD}}_j^i\left[n\right]$ as follows:

$${\text{pFD}}_j^i=\left\{\begin{array}{cc}{\text{FD}}_j^i,\hfill & {\text{FD}}_j^i>{\mathrm{\mu }}^i+{\mathrm{\sigma }}^i\hfill \\ \text{1.0,}\hfill & \text{elsewhere}\hfill \end{array}\right.\text{,}i=1,\dots ,L_1;j=1,\dots ,M,$$

(12)

where ${\mathrm{\mu }}^i=\text{mean}\left({\text{FD}}_j^i\right)$ is the mean value of the ${\text{FD}}_j^i$ vector, ${\mathrm{\sigma }}^i=\text{std}\left({\text{FD}}_j^i\right)$ is the SD of the ${\text{FD}}_j^i$ vector, and L₁ is the number of the self-adjusted iterations. Thus, the vector $z^i={\text{FD}}_j^i-{\text{pFD}}_j^i+{\mathrm{\mu }}^i$ is created, and 1.0 is the minimum value of the estimated FD sequence. The iterative procedure stops when the following stopping criterion is satisfied:

$$\text{S}{\text{C}}^i=\left|E\left\{{\left(z^i\right)}^2\right\}-E\left\{{\left(z^{i-1}\right)}^2\right\}\right|<\text{acc},i=1,...,L_1,$$

(13)

where E{}, as in the former stopping criterion, denotes the expected value, the parameter acc is a small positive number (0<acc1.0) that corresponds to the desired accuracy in the procedure, and the initial value of z⁰ is equal to 0. When the stopping criterion is not satisfied, the vector ${\text{FD}}_j^i$ is replaced by the vector zⁱ, and it continues the iterative procedure. When the stopping criterion is satisfied, the FD-PPA generates the ${\text{FDPP}}_j^k$ sequence of the j-th WT coefficient as follows:

$${\text{FDPP}}_j^k={\displaystyle \sum _{i=1}^{L_1}}\text{pF}{\text{D}}^i-\left(L_1-1\right),k=1,...,L;j=1,...,M,$$

(14)

where L is the iteration number of the procedure that is described in Section “WT–FD Iterative Procedure.” After the FD-PPA implementation, the small peaks that do not correspond to any sound and their duration is less than int(0.015F_s), and their normalized amplitude less than 0.25 are removed. Again, int() indicates the integer part of the argument, the constant is empirically set at 0.015, and F_s denotes the sampling frequency of the signal. Subsequently, the ${\text{FDPP}}_j^k$ sequence is generated and thereafter two binary thresholds are constructed, as shown in Figures ​Figures2D,E.2D,E. The first binary threshold, i.e., ${\text{SBTH}}_j^k$ is used for segmenting the WT coefficients that are related to the desired signal, while the second one, i.e., ${\text{NBTH}}_j^k$ is used for segmenting the WT coefficients that are related to the background noise. These two binary thresholds are defined as follows:

$${\text{SBTH}}_j^k=\left\{\begin{array}{c}1,{\text{FDPP}}_j^k\ne 1\hfill \\ 0,{\text{FDPP}}_j^k=1\hfill \end{array}\right.\text{,}$$

(15)

$${\text{NBTH}}_j^k=\left[\text{1}-{\text{SBTH}}_j^k\right]\text{,}k=1,...\text{,}L\text{;}j=1,\dots \text{,}M\text{,}$$

(16)

The multiplication of the ${\text{SBTH}}_j^k$ with the WT coefficient gives a set of de-noised signals that create the $X_S^k\left[n\right]$ vectors as defined in Section “WT–FD Iterative Procedure,” while the multiplication of the ${\text{NBTH}}_j^k$ with the WT coefficients gives the set of the $X_U^k\left[n\right]$. Figures ​Figures2A–E2A–E gives an example where a working scheme of the proposed method is presented on the third WT level of an input signal. It shows that the FD method successfully detects the location of the sounds by using the binary sequences, and it separates the non-stationary bioacoustics signal from the stationary background noise.

In this study, the final goal is to segment the fHS and separate the S1 from the S2 beats. The decision between S1 and S2 is based on the fact that in a cardiac cycle the diastolic duration is longer than the systolic one (Papadaniil and Hadjileontiadis, 2014). For that reason, the following inequality is checked:

$$S\left(2i+1\right)-S\left(2i\right)<S\left(2i+3\right)-S\left(2i+2\right),$$

(17)

where S(l) is a vector that is created by the binary threshold ${\text{SBTH}}_j^k$, and it contains the locations of the start and the end of every fetal heartbeat. Moreover, i=1,…,(N₁/2)−2, where N₁ is the length of the S(l) vector. If Eq. 17 is true, the interval [S(2i+1):S(2i+2)] corresponds to S2, otherwise, it corresponds to S1. The first and the last heartbeat of the signal are not determined from this inequality. Hence, they need to be separately defending. For i =1, if Eq. 17 is true, then the second sound [S(4):S(5)] is S2 and the first sound [S(1):S(2)] is S1. Respectively, for i =(N₁/2)−2, if Eq. 17 is true, the last sound [S(N₁−1):S(N₁)] is defined as S1.

A criterion of each estimated fetal heartbeat amplitude and the distance between fetal heart cycles is also considered for better decision between S1 and S2 beat. In the literature, the mean amplitude of a fetal S1 beat is equal to 0.7 (Cesarelli et al., 2012), and the distance between fetal heart cycles, i.e., between S2 and the following S1, depends on the fHR. The smaller distance between fetal heart cycles is in case of tachycardia and is about 140ms. Thus, for the decision between S1 and S2 beat, the S1 estimated beat must surpass the 0.5 normalized amplitude and the S2, S1 inter-distance must be outdistance within 130ms.

Implementation and Evaluation Issues

The analysis of this study was applied on a personal computer using Matlab R2015a and tested on simulated databases. Every input signal was tested for 10s considering F_s =1,000Hz, i.e., 10,000 samples.

For the purposes of this research and the algorithmic development of the WT–FD method, a database with fPCG signals was created. Each signal contains simulated S1 and S2 auscultation sounds created by Hadjileontiadis using the model of Chen et al. (1997) and Xu et al. (2001) and adjusted to the duration of fetal heartbeat. The inter-distance between S1 and S2 heart sounds is given by the expression SSID=210−0.5fHR according to Kovacs et al. (2000). Moreover, in order to represent the noise presence, additive white Gaussian noise was used, resulting in signal-to-noise-ratio (SNR) within the range of SNR=[8, 3]dB. The SNR values were computed according to the following steps; measure the power of the signal (P_s), convert the given SNR in decibels (SNR_dB) to linear scale according to $\text{SN}{\text{R}}_{\text{linear}}={10}^{\frac{\text{SN}{\text{R}}_{\text{dB}}}{10}}$, and finally create the noise vector from Gaussian distribution of specific noise variance according to $\text{noise}=\sqrt{\frac{P_S}{\text{SN}{\text{R}}_{\text{linear}}}}\cdot \text{random}$, where random is a vector of normally distributed random numbers with the signal length.

The database consists of signals with different heart conditions corresponding to cases such as tachycardia, bradycardia, and arrhythmia. Specifically, after the 20th week of gestation, the fHR is stabilized between the 110 and the 160bpm. Thus, for the normal heartbeat signals, the fHR was set at 140bpm, for the bradycardia signals at 110bpm, for the tachycardia signals at 180bpm, and for cases of arrhythmia a range of 80–200bpm was considered. Hence, many signals with different conditions and different values of additive white Gaussian noise were created and used for testing the present study.

Furthermore, for better assessment of the WT–FD technique, the method was tested on the simulated fPCGs database available via PhysioBank.¹ PhysioBank is a large archive of digital recordings of physiological signals and related data for use by the biomedical research community (Goldberger et al., 2000). The simulated fPCG database was created by Cesarelli et al. (2012) and Ruffo et al. (2010). This data set is a series of synthetic fPCG signals related to different fetal states and recording conditions. Simulated fPCG were generated as a sequence of frames, each of which includes simulated S1 and S2 signals, corrupted by noise. These signals are qualified by a range of SNR values that were computed in decibels according to the following formula:

where P_s and P_n are the power of fHS and the power of the noise, respectively. The noise source was simulated by generating maternal and fetal noise, maternal first heart sound, white Gaussian noise, environmental noises, and limited duration impulses considering as sensor noises. The epoch lengths were set equal to 430 and 400ms for the analysis of the PCG drawn from the two databases, respectively, using in both databases a window length of 50 samples.

Results

As mentioned in Section “General Evaluation Indices,” the WT– FD technique was tested on two simulated databases of fPCG signals. The results of this assessment are presented in Tables ​Tables11 and ​and22 where the Q_P, D_R, and S_F indices are tabulated, providing a means for the evaluation of the performance of the WT–FD algorithm for the detection of the fetal S1 and S2 heartbeat and also each fHS separately.

In particular, Table ​Table11 presents the cases of 12 simulated fPCG signals created for the present study and consists 4 different fHRs and 3 different SNR values (white Gaussian noise). From Table ​Table11 it is clear that the WT–FD method is efficient for multiple conditions. The Q_P index indicates that in all cases of fHR the WT–FD correctly predicts almost all the observed sounds in different SNR values up to 3dB. Specifically, in cases of normal fHR (140bpm), the algorithm has mean performance 100%. In cases of tachycardia (180bpm), bradycardia (110bpm), and arrhythmia, the efficiency of the method is slightly lower although it is sufficiently effective in the detection of the S1 beat locations.

Moreover, Table ​Table22 presents the cases of seven simulated fPCG signals from PhysioBank with different SNR values. Results for the cases of normal fHR with a range of SNR noise lying in [−26.3, −6.6dB] demonstrate that the WT–FD algorithm segments and detects almost all the observed heart sounds and has a mean accuracy 81%. However, it is clear that the lower the SNR value, the harder it is for the WT–FD to segment and select the correct S2 fetal heartbeat. Very low SNR (less than −22.1dB) makes the S2 sound difficult to distinguish from the noise. The D_R index declares that, despite the fact that the algorithm misses a few heartbeats, it does not detect false locations. Most of the detected sounds are assigned to real fetal heartbeat locations. Furthermore, it is notable that all the detected S1 beat locations refer to real sounds.

Figure ​Figure33 shows the efficiency of the WT–FD technique to recognize the fHS in signals with unexpected noise presence. Figure ​Figure3A3A corresponds to the X[n] unprocessed signal, and Figure ​Figure3B3B corresponds to the X_REC[n] segmented reconstructed signal. In X[n] signal it is obvious that there is a noisy segment, which is marked with an arrow, that masked the S2 heart sound. In the X_REC[n] signal it is clear that the WT–FD successfully extracts the sound.

Open in a separate window

Figure 3

Experimental result from the application of the wavelet transform–fractal dimension scheme to simulated fetal phonocardiograph signal. (A) X[n] represents a section of 5,000 samples of a normal heart rate case with unexpected robust noise. (B) X_REC[n] corresponds to the normalized treated signal without the overlap of noise. The arrows indicate the location of the S2 sound that the algorithm efficiently reveals.

The proposed WT–FD approach was also tested in real fPCG signals from a small pilot study, involving recordings from three pregnant women. The fPCG signals were recorded using vibration sensors (cost $1 each) embedded in high definition 3D-printed plastic harnesses. Each harness holds a ceramic piezo vibration sensor (35mm diameter) on the maternal abdomen with rubber-made cushion to minimize the shear noise. The 3D-printed harness is designed with precise parameters that rigidly mount the piezo sensor. Each sensor picks fPCG signals through a coaxial cable having very high insulating resistance. Power lab data acquisition system by AD instrument² was used to record the abdominal phonograms at a sampling frequency of f_s =1,000Hz.

A characteristic example of one channel fPCG recording (time section of 3s) with maternal heart rate of 96bpm and fHR of 145bpm is shown in Figure ​Figure4A.4A. From the latter, it is clear that the fPCG signal is modulated by noise from various sources, and the most intense interferences are the mother’s respiratory and heart sounds. Figure ​Figure4B4B shows the fourth level of the estimated WT coefficients from the eight level WT decomposition. The WT–FD method selects these WT coefficients that include information regarding the signal of interest, i.e., fHSs, based on the criterion (Eq. 9). For the real fPCG data processing, the constant of the criterion was set at 0.001, leaving out the first three decomposition levels and including only those with embedded fHSs. Finally, Figure ​Figure4C4C depicts the estimated fHS signal, i.e., the detected S1 and S2 fHSs, marked with (S1) and (S2), respectively, as the final output from the proposed WT–FD method. Note that, in some cases [Figure ​[Figure4C4C around (0.5–1.5s)], three S2 fHSs were missed by the WT–FD filter due to their lower intensity, compared to the neighboring S1 ones and the local background noise. Nevertheless, when comparing the original recording of Figure ​Figure4A4A with the outputted fHS signal from the WT–FD approach in Figure ​Figure4C,4C, a clear contribution to the enhancement of the fHS signal from its original recording is evident.

Open in a separate window

Figure 4

Analysis results when wavelet transform (WT)–fractal dimension (FD) filter is applied to a part of real data. (A) A time section of 3s of the real fetal phonocardiograph recording, with maternal heart rate of 96bpm and fetal heart rate of 145bpm. (B) The fourth level of the estimated WT coefficients selected for the detection of the fetal heart sounds (fHSs). (C) The result of the de-noised fHS signal after the final WT–FD analysis with S1 and S2 denoting the first and second fHS, respectively.

It should be noted that the fHSs are not perfectly periodic due to the heart rate variability. It can be seen that, despite the noisy signal, the WT–FD method successfully identifies the fHSs and their time location and duration, giving the physicians the means to estimate the fHSs and the fHR. The wavelet morphology of the sounds could vary with different pathophysiological conditions. This is of great importance when the fHSs are continuously recorded for long-term analysis.

Concluding Remarks

The fPCG signals are of low amplitude and loaded with heavy noise. The sources of the noise, i.e., maternal sounds, fetal movement, sound produced by the transducer, and other, are overlapping the main fHS. The literature in the area of fetal auscultation is not strict about the intensity of background noise and the intensities of S1 and S2 heartbeat, because of the different auscultation devices but also due to the different gestation age. Nevertheless, it is possible to argue that the amplitude of the stationary background noise did not fully overlap the fHS, and that the SNR values that have been tested in the present study were sufficient samples of heavily loaded signals. However, as it was shown by the testing results, the WT–FD scheme is quite satisfactory in the analysis of the fHS. This first approach of the research in fPCG signals reveals sufficient information, which indicates that this technique can be a promising fHS segmentation tool. Furthermore, there are perspectives for low-cost and continuous recordings in homecare setups and diagnosis of conditions related, for example, to fetus maturation or specific abnormalities.

Future work will focus upon the extension of WT–FD to real recorded signals for a better review of fetal functionality and the fetal heart cycle. Moreover, multichannel recordings could be considered, taking into account the spatial orientation of the fetus and the proximity to the mother’s heart sound noise. As phonocardiography has been an important field in the research area related to the fetus for some time, efficient characterization of fetal heartbeat could contribute to the automated determination of fetus parameters. In this vein, the determination of multiple fetus health data may reveal new aspects, which could improve the safety of pregnancies.

Author Contributions

Producing the code used in the paper: EK and LH. Analyzing the signals and drawing conclusions as well as discussing the structure of the paper and writing up the final version: EK, LH, IC, and AK.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer, WT, and handling editor declared their shared affiliation, and the handling editor states that the process nevertheless met the standards of a fair and objective review.

Acknowledgments

Footnotes

References