Vol.2, No.6, 380-389 (2009)
SciRes Copyright © 2009 Openly accessible at http://www.scirp.org/journal/JBISE/
Development of a low cost fetal heart sound monitoring
system for home care application
Arun Kumar Mittra1, Nitin K. Choudhari2
1MIET, Gondia: Department of Electronics Engineering, Manoharbhai Patel Institute of Engineering and Technology, Gondia (MS),
India; 2Smt. Bhugwati College of Engineering, Nagpur, India.
Email: akmittra@gmail.com
Received 6 January 2008; revised 26 April 2009; accepted 26 June 2009.
Variations in fetal heart rate (FHR) is a potential
indicator of stress on unborn in the womb of
mother. In hospitals, FHR surveillance is per-
formed by ultrasound based Doppler equip-
ments. However, recent studies show that fre-
quent exposure to ultrasound radiations is not
recommended for the fetal well-being. Because
of this and many other reasons, these instru-
ments are not recommended for prolonged
home monitoring applications. This work is fo-
cused around development of a prototype sys-
tem for fetal home monitoring application. Pre-
sented system can record the abnormal FHR
and alert the pregnant women to report to a
physician. Recorded data is then processed by
a novel methodology for deriving results of di-
agnostic importance. The instrument has been
tested on pregnant women in the clinical envi-
ronment and has gone through an extensive
clinical trial at local hospitals. The results show
that the technique is suitable and effective for
long-term FHR home monitoring application.
Keywords: Ambulatory Monitoring; Phonocardio-
graphy; Cardiography; Simulation; Signal Processing
To diagnose pre-term labor, ambulatory monitoring for
abnormal FHR has proven to be an effective method [1].
Abnormality in fetal heart rate (FHR) is an indicator of
pre-maturity and miscarriage. It is very important to
monitor, such abnormalities in pregnant women, which
are at high risk, with history of miscarriage. These ab-
normalities are unpredictable and may occur at any time,
especially in the case of pre-term labor. The pathogene-
sis of pre-term labor is still poorly understood, however,
the unusual occurrence of pre-maturity and miscarriage
can be largely prevented by the timely diagnosis of
pre-term labor and its arrest with toco lytic medication. If
the unborn heart rate increases very high or drops to a
very low, it calls for urgent attention. In both the cases, it
is obvious that the baby is in stress and special urgent
medical attention is needed. For this reason, the elec tronic
monitoring of the FHR has become one of the most fa-
miliar methods used in the antenatal period [2,3]. The
ultrasound based Doppler instruments are widely used
for this purpose in hospitals, but for varied reasons, they
are not suitable for home monitoring application and
long-term surveillance of unborn [4,5]. It is imperative
to note that these instruments are also invasive in nature.
There is still a gap between existing technologies and
the user requirement for safe, convenient, and reliable
fetal monitoring [6,7 ]. In view of these considerations, a
strong need is felt for the development of a FHR moni-
toring machine which will be non-invasive, cost effec-
tive, simple to operate and which can be used by a preg-
nant woman for prolonged home monitoring application
[8]. Preliminary experiments in this regard have been
conducted [9,10] and a cost effective prototype is de-
veloped for at home long term FHR recording and
monitoring. This instrument can record fetal heart sound
for hours in a standard MP3 format. In case of abnormal
symptoms, the subject visits the physician where the
recorded data can be analyzed with the help of associ-
ated software and computer. The audiovisual display of
fetal heart sound will provide valuable additional infor-
mation to the physician for diagno sis and treatment.
The basic technique used in the presented instrument
is called fetal Phonocardiography (fPCG) [11,12]. In this
technique a specially designed microphone is placed on
the abdomen of the subject, which primarily detects and
records the fetal heart sound. However, phonocardio-
graphy is extremely suscep tible to ambient noise [13,14,
15]. Unfortunately, the fetal heart activity produces
much less acoustic energy and moreover it is surrounded
by highly noisy environment. This noise has a direct
consequence on the signal that often changes remarkably
from one beat to next, with the additional characteristic
A. K. Mittra et al. / J. Biomedical Science and Engineerin g 2 (2009) 380-389
SciRes Copyright © 2009 Openly accessible at http://www.scirp.org/journal/JBISE/
of poor signal to noise ratio (SNR). These unwanted
disturbance signals contribute an additional difficulty in
the detection of the principal fetal heart sounds (S1 and
S2). Hence it is necessary to develop and evaluate sig-
nal-processing technique to improve SNR before ob-
taining reliable time references of the fetal heart sound
signal [16]. In order to improve SNR, and accomplish an
optimal external noise cancellation performance, phono-
cardiographic devices need a very superior and advanced
noise reducer in the preprocessing stage of the instru-
ment. In this work a very effective noise reducer is de-
signed and used in the presented prototype system. The
design is implemented with help of Matlab Simulink and
intensive tests are carried out in order to evaluate the
performance of resultant method. The instrument has
been found effective from 30th week to final term of
gestation with satisfactory level of sensitivity.
The paper is organized as follows: The next section
discusses the hardware of the presented system, used for
the detection and recording of fetal heart sound and am-
bient noise. The following section, describes software
and different signal processing techniques used by the
system. The article then presents a comprehensive com-
puter simulation. The last section deals with experimen-
tal and clinical trial results with conclusions.
The Fetal heart sound monitoring system presented in
this study, primarily comprises of two main modules:
1) Detection and Recor ding Module (DRM).
2) Processing and Display Module (PDM).
The DRM is a small hardware, which is placed on the
subject’s abdomen for detection and recording of the
fetus heart sound signal. This module records the ab-
normal FHR and alerts pregnant woman, that it is time to
seek some medical attention.
The PDM is software, developed for physician’s per-
sonal computer. When the subject approaches the physi-
cian along with DRM, recorded data is down loaded and
then processed with the use of PDM, to generate the
results of diagnostic importance.
2.1. Detection and Recording Module (DRM)
It is a small low cost hardware comprising of a specifi-
cally designed acoustic cone, piezoelectric sensor, pre-
amplifiers, power amplifiers, filters and a USB com-
patible MP3 voce recorder. The basic functions of this
module are as follows:
1) Fetal heart sound detection from maternal abdo-
men and recording of the same on one of two
channels in MP3 voice recorder (Abdominal
2) External noise detection through an open-airmi-
crophone and recording on another channel (Noise
3) Generation of audiovisual indications from fetal
heart sound signal.
Block diagram of DRM with all necessary details is
shown in the Figure 1. It can be seen that the complete
Figure 1. Block diagram of DRM hardware.
A. K. Mittra et al. / J. Biomedical Science and Engineerin g 2 (2009) 380-389
SciRes Copyright © 2009 Openly accessible at http://www.scirp.org/journal/JBISE/
system is divided in two basic sections: a) Abdominal
Channel b) Noise Channel.
a) Abdominal Channel: Fetal phonocardiography re-
quires the conversion of mechanical vibration on sub-
ject's abdomen to electrical signal by microphone. Fetus
heart sound is extr emely weak hence it cannot be sensed
properly by putting a sensor immediately on the subject's
abdomen. To overcome this problem a particular acous-
tic cone is developed which is a direct extension of the
chest piece of standard stethoscope. The air enclosed in
the cone acts as a transmission media between the mem-
brane and electro-mechanical transducer device. The
output of the sensor is fed to pre-amplifier for high am-
plification and better no ise rejection. IC LM 381 is used
here for this particular purpose, which raises the signal
from the transducer level to the line level.
It is essential to keep ambient noise as low as possible;
this is carried out by an active low pass filter with
cut-off frequency of 200 Hz. This value of cut off fre-
quency is selected, because most of the fetus heart sound
spectrum lies below this frequency limit. Active filter is
implemented using an easily available operational am-
plifier IC 741 along with suitable resistor capacitor net-
work. IC TBA810 based power amplifier further streng-
thens the output signal from filter, and provides audible
fetus heart sound. I t is then recorded on any one chann el
of MP3 voice-recorder. This amplifier further provides
driving power to audio-visual indicator of the DRM de-
b) Noise Channel: In fetal phonocardiographic meas-
urement, ambient noise creates major problem at signal
processing stage [17,18]. To overcome this problem,
special signal processing techniques are used in this
study, which require a primary sample of the noise, cre-
ating disturbance in the signal of interest. To facilitate
this, ambient noise is detected through an independent
open-air microphone. After pre-amplification and filter-
ing, noise signals are recorded on another channel of the
memory device.
It is important to note that abdominal channel micro-
phone detects the sound primarily originating from the
maternal abdomen, but these signals get mixed with a
damped version of the external noise. The open air Noise
Channel microphone detects only the ambient noise and
does not contain any traces of fetal heart sound signal.
Photographs of prototype experimental model are shown
in the Figures 2(a) & 2(b).
2.2. Processing and Display Module (PDM)
The PDM is the software part of presented system, made
available in the physician’s personal computer. When
pregnant woman feels some abnormality and the DRM
alerts her, she may go to the hospital with the DRM.
Stored data within the device is used for further proc-
essing and investigation through PDM. Block diagram of
PDM with all necessary signal flow details is shown in
the Figure 3.
A brief description of each signal-processing module
incorporated in the block diagram is given below:
a) Data Acquisition: The output of the MP3 voice re-
corder is directly fed to the Line-in of the multimedia
card, which contains on board signal conditioning, ana-
log to digital conversion and digital signal processing
hardware. Matlab data acquisition toolbox is used to
download both channels of MP3 voice recorder and to
store it in two separate *.wav files. These files carry
noised fetal heart sound signal and external noise sepa-
b) Adaptive Filter: Adaptive filters track the dynamic
nature of a system and allow elimination of unwanted
part of the signal. In this study adaptive filters are used
for external noise cancellation i.e. removal of external
unwanted background sound signal from the fetal heart
(a) (b)
Figure 2. Photographs of prototype.
A. K. Mittra et al. / J. Biomedical Science and Engineerin g 2 (2009) 380-389
SciRes Copyright © 2009 Openly accessible at http://www.scirp.org/journal/JBISE/
Figure 3. Block diagram of PDM.
sound signal. As shown in Figure 3, desired signal d(k),
the one to clean up, comes from abdominal channel,
carries noise n(k) and wanted signal s(k). The Noise
Channel carries only background noise n'(k), is applied
as input signal x(k) of the filter. As long as the input
noise to the filter n'(k) remains correlated to the un-
wanted noise n(k), the adaptive filter adjusts its weights
w(k) to reduce the value of the difference between y(k)
and d(k), this results in elimination of ex ternal noise and
a clean fetal heart signal s(k) will appear on the error
port. The generalized mathematical relationship be-
tween different signals and filter weights can be de-
picted as:
d(k) = s(k) + n(k)
y(k) = Filter{x(k), w(k)}
or y(k) = w(k) . n’(k)
e(k) = d(k) – y(k)
= s(k) + n(k) – w(k) . n’(k)
w(k+1) = w(k) + e(k) x(k)
Notice that in this implementation, the error signal
actually converges to the input data signal, rather than
converging to zero.
c) Band Pass Filter: Adaptive filtering of recorded
signal removes only the external noise from the compos-
ite abdominal signal. A band pass digital filter is de-
signed to limit the signal in 35 Hz < f < 200 Hz fre-
quency band. The selection of lower frequency limit is
based on the experimental result, that a considerable part
of the disturbing maternal heart and digestive sound lies
below this border, while the fetal heart sounds are not
dominantly present there any more. Upper limit is set to
remove maternal respiratory sound and remaining traces
of external noise signals.
d) Phonocardiogram: Fetal Phonocardiograph is a
time versus amplitude plot of fetal heart sound and is
considered as the primary time domain characteristic of
the fetal heart sound signal. It is a graphical representa-
tion of vibration or sound signal detected from the ma-
ternal abdominal wall, caused by the contractile activity
of the fetal heart. The general fPCG wave pattern of the
signal over cardiac cycles may be readily appreciated by
visual inspection and can be used as a potential indicator
of few congenital diseases.
e) Spectrogram: fPCG signals posses multiple reso-
nance frequencies. This leads towards the need to de-
scribe the fPCG signals, not only in terms of time but
also in terms of frequency domain, also known as spec-
trogram. It provides distribution of the signal’s energy
or power over a wide band of frequencies.
f) Envelop Generation: In order to find out exact pe-
riodicity of heartbeat, it is proposed to use env elop of the
fetal heart sound signal. Th is envelope encompasses and
traces the peaks of signal under consideration. In this
application it is derived by the method of squaring and
low pass filtering [19]. The resultant envelope signal
reflects the amplitude dynamics of signal on the same
time scale as of the original signal.
g) Burst Generator: After the process of envelope
generation, exact positioning of amplitude burst is car-
ried out by the method of thresholding and relaying.
Whenever amplitude of signal envelops goes above a
pre-defined limit it is relayed over as a maximum value
otherwise it is taken as zero. This process results in a
series of discrete pulses representing occurrences, and
duration of fetal heartbeat s.
A. K. Mittra et al. / J. Biomedical Science and Engineerin g 2 (2009) 380-389
SciRes Copyright © 2009 Openly accessible at http://www.scirp.org/journal/JBISE/
h) FHR Calculation: The last element of the sig-
nal-processing block is for the FHR determination. This
is carried out by counting numbers of discrete pulses in
the signal for a pre-defined time interval.
Processing and Display Module (PDM) described above
is simulated using the Simulink modeling tool of Matlab
version 7. The model is built by interconnecting requi-
site blocks, which are available in the software library
and their parameters are entered while designing them
for simulation. The inputs of the simulating system are
the recorded signals of DRM module, where as the out-
puts are Phonocardiogram, Spectrogram and record of
FHR in Beats Per Minutes (BPM). System simulation of
developed PDM module is shown in Figure 4.
After data acquisition, signals are de-noised with the
help of adaptive filter. Fetal heart sound signals, detected
from mother's abdomen are fetched from corresponding
*.wav file and applied to the desired port of the filter
block. This signal carries fetal heart sound and a damped
version of the external noise. The unwanted external
noise is available in another *.wav file and is applied to
the input port of the filter. De-noised signal comes out
through the error port of the filter block. It should be
noted that the output port is not used in the simulation
process; however this port provides internal feedback for
error calculation in the filter block.
Adaptive filtering eliminates only the external noise.
For suppressing remaining artifacts, digital band pass
filter is used in the simulation. This block allows signals
of the specified range only, while signals of all other
frequencies are attenuated. After the band pass filtering
signals are relatively less distorted and applied to the
time scope block of the module. This block provides the
time-domain response of the fetal heart signal, which is
also called as phonocardiogram. The frequency domain
response i.e. cardio-spectrogram of de-noised signal is
produced through periodogram and vector scope block
icons. The periodogram block computes a non-paramet-
ric estimate of the spectrum. The block averages the
squared magnitude of the FFT computed over windowed
sections of the in put and no rmalizes the spectral average
by the square of the sum of the window samples. The
vector scope block is a comprehensive display tool
similar to a digital oscilloscope. It is used here to plot
frequency-domain response of the de-noised fetal heart
sound signal.
Figure 4. System simulation of PDM.
Figure 5. Sub-system for envelop generation.
A. K. Mittra et al. / J. Biomedical Science and Engineerin g 2 (2009) 380-389
SciRes Copyright © 2009 Openly accessible at http://www.scirp.org/journal/JBISE/
Figure 6. FHR calculator sub-system.
The simulation block diagram showing the bottom-
most blocks (refer Figure 4) are related with the FHR
calculation. The first block is for envelope generation
and it is a simulink subsystem as shown in Figur e 5.
This sub system is based on the concept of squaring
and low pass filtering. The input signal is first multiplied
by itself. Squaring the signal effectively modulate the
input by using itself as the carrier wave. This means half
the energy of the signal is pushed towards higher fre-
quencies and half is shifted towards DC. The envelope
can then be obtained by keeping all the DC low fre-
quency energy and eliminating the high frequency en-
ergy. In this sub-system, a simple minimum-phase low
pass filter is used to get rid of the high frequency energy.
Output of envelope generator is connected to the relay
block (refer Figure 4). This block allows its output to
switch between two threshold values. Once the relay is
ON, it remains ON until the input drops below the valu e
of the switch-off point parameter and when the relay is
OFF, it remains OFF until the input exceeds the value of
the switch-on point parameter. This block converts the
envelope signal into a series of discrete pulses. These
pulses are fed to FHR Calculator sub-subsystem. Details
of this sub-system are shown in Figure 6, in which the
Counter block increments an internal counter each time
it receives a trigger at the click (Clk) port. A trigger sig-
nal at the reset (Rst) port brings the counter to its initial
Counter output is converted to Beats Per Minute (BPM)
and is then fina l l y di spl aye d on t he o utput device.
Experimental testing is necessary to verify the reliability
and performance of any system under a developmental
stage. An artificial womb, which give simulated testing
conditions for fetal monitoring system is very appropri-
ate and useful for initial testing in comparison to actual
clinical testing on pregnant women. In this work, an arti-
ficial womb is prepared for simulated performance test-
ing of monitoring systems under study. Through Matlab
signal processing toolbox, simulated signals are gener-
ated for fetal heart sound, maternal heart sound, maternal
respiratory sound and for external noise. After amplifi-
cation these signals are applied to different speakers
placed underneath a rubber balloon filled with water.
DRM hardware under test is placed on the opposite side
of the balloon. This arrangement simulates a fetal heart-
beat passing through amniotic fluid to the wall of the
mother's abdomen. For providing external support, the
complete assembly is housed in a solid wooden tub. It is
then placed in a thick glass envelop for elimination of
outside noise interference. Presented system was initially
tested on above-mentioned artificial womb. This was a
totally subjective test, performed only to check viability
of the instrument.
After satisfactory performance with artificial womb,
system has been tested on the pregnant women in clini-
cal environment. More than 15-fetal heart sound re-
corded samples were taken from different women, who
were between 36 to 40th week of singleton pregnancy.
Recorded data was transferred to personal computer in
*.wav file through multimedia card. Matlab Simulation
discussed above was used to process and display the
recorded sound from wave files. Figure 7 shows signal
waveforms at various stages of the simulation, obtained
from abdominal recording of a pregnant woman at 39
weeks of gestation (Subject No. 1). In these graphs, X-
axis represents the time in seconds whereas Y-axis
represents amplitude of signal in volts. Uppermost wave-
form (Graph a) represents 2 seconds time span of repre-
sentative sample of fetal heart sound, practically re-
corded through the abdominal microphone.
Second waveform (Graph b) is the corresponding ex-
ternal noise, recorded through external microphone. It
may be noted that noise level is very high, which con-
taminates the fetal heart sound to a larger extent. This
representative sample of noise is used as a reference
input for adaptive filters in signal processing stage of
simulation. Third waveform (Graph c) describes the
de-noised signal coming out from the adaptive filter. It
can be observed that external noise is considerably re-
duced and amplitude burst are distinctly visible in the
waveform. In order to further increase the signal to noise
ratio, band pass filters are used. Fourth waveform
(Graph d) is the signal after band pass filtering and this
A. K. Mittra et al. / J. Biomedical Science and Engineerin g 2 (2009) 380-389
SciRes Copyright © 2009 Openly accessible at http://www.scirp.org/journal/JBISE/
Figure 7. Simulation output (a) Signals from abdominal channel (b) Signals from external channel (c) Signal after adaptive filtering
(d) Signal after band pass filtering (e) Signal after envelope generator (f) Signal after thresh holding.
is the final phonocardiographic signal, which the instru-
ment provides. Fif th waveform (Graph e) is signal enve-
lope provided by the complex process of envelope gen-
eration. This envelope is then passed through an ampli-
tude thresholding process that in turn converts amplitude
burst of the envelope signal into discrete pulses. The last
waveform (Graph f) indicates the final processed signal,
used for the FHR calculation.
It is believed that a more precise examination of
phonocardiographic signal may be useful for pre-detec-
tion of intra uterine growth retardation and other abnor-
malities of fetal. To facilitate this, a separate time scope
block is provided in the simulation. This block provides
a zoomed-in version of signal for any specified period of
A. K. Mittra et al. / J. Biomedical Science and Engineerin g 2 (2009) 380-389
SciRes Copyright © 2009 Openly accessible at http://www.scirp.org/journal/JBISE/
Figure 8. Fetal phonocardiogram.
Figure 9. Fetal spectrogram.
time. Figure 8 shows such display for a time interval of
0.8 seconds.
Phonocardiogram is the time domain response of the
fetal heart sound signal. Presented system is in addition;
capable of providing frequency domain response of the
signal, called Spectrogram. It represents the contribution
of every freq uency of the sp ectrum to the power of over-
all signal. Spectrogram of fetal heart sound for a small
time window around 0.55 sec instant of previous illus-
tration is shown in the Figure 9.
In order to support the performance of developed sys-
tem, phonocardiogram signals recorded through proto-
type were compared with signals of simultaneously used
ultrasound Doppler based instrument (Model: Coddle-
Graph–L of Maestros Mediline Systems). In this com-
parative experimentation following parameters were
N’ = Total number of amplitude bursts detected by
N = Total number of amplitude bursts detected by ref-
erence instrument.
M = Total number of missed bursts by the prototype.
F = Total number of false bursts detected by the pro-
A. K. Mittra et al. / J. Biomedical Science and Engineerin g 2 (2009) 380-389
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Table 1. Outcome of comparative measurement.
Subject No Number of Amplitude
Burst from Prototype Number of Amplitude Burst
from Reference Instrument Number of Missed
Amplitude Burst Number of False
Amplitude Burst Performance
Index %
1 260 270 10 0 96.30
2 290 286 0 4 98.60
3 274 280 6 0 97.86
4 278 284 6 0 97.89
5 310 302 0 8 97.35
6 294 298 4 0 98.66
7 290 296 0 6 97.97
8 318 324 0 6 98.15
9 292 290 2 0 99.31
10 312 302 10 0 96.69
11 266 280 0 14 95.00
12 302 306 0 4 98.69
13 286 288 0 2 99.31
14 298 304 0 6 98.03
15 314 310 4 0 98.71
16 304 308 0 4 98.70
From these data, performance of the instrument [20]
can be derived with the help of formula:
Performance Index (NMF
X 100
This comparative measurement is performed on 16
pregnant women between 36th to 40th week of gestation
age, and average recording duration was stayed limiting
to one minute. It is observed that in most of the cases,
phonocardiographic-based prototype signal quality is
almost at par with the ultrasound Doppler based signals.
Table 1 shows result of these measurements and calcula-
tion of corres p on d i n g perform a nce indi ces.
In light of these recorded values, the overall Perform-
ance Index of the system is found around 97% in corre-
lation of ultrasound based Doppler instrument. This per-
formance value is fairly good for a prototype model and
will certainly improve in commercially advanced sys-
This work presents development of a very powerful,
non-invasive, portable and low cost battery operated
standalone fetal heart sound recording and monitoring
system that can be used in prevailing home environment.
The hardware of prototype model is of the size 9 X 8 X
4.4 cm and of the weighs 205 grams with 9 V alkaline
battery. Signal recorded through this prototype model
are digitally processed and analyzed on a personal com-
puter. Using enhanced adaptive and band pass filtering
techniques, a remarkable imp rovement in signal to noise
ratio is achieved by the system. Processed signals are
finally used to produce impressive results of significant
diagnostic and clinical importance. Instrument has been
tested on pregnant women with varied gestational period
and also validated by simultaneous measurement with a
standard ultrasonic Doppler device. From the results it
can be concluded that the presented system is viable and
can effectively be used in the development of commer-
cial phonocardiographic-based fetal home care monitor-
ing system.
The fetal heart sound recordings were done at district government
women hospital and at Ratnaparkhi Nursing Home Gondia (M.S.). The
authors of this paper would also like to thank Dr. Shrish Ratnaparkhi
(Gyneacologist), Dr. (Mrs.) Megha Ratnaparkhi (Obstetrician), Prof.
Vijay Chourasia and Prof. (Mrs.) Vijaya Rahangdale for their kind
support in carrying out observations with the help of developed proto-
type instrument. Pregnant women who volunteered to participate in
clinical test are also appreciated for their kind gesture.
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