Though the reflected signal is used to

Though the first
observation of foetal heart sound was reported during the 17th, it was Lejumeau
Kergaradec who reported the usefulness of the auscultation in diagnosis of
twins, foetal lie and presentation during the pregnancy 1. Foetal heart rate
is an important parameter in the assessment of foetal wellbeing both
intrapartum and antepartum, Kilian was the first to propose that changes in
heart rate can be used to diagnose foetal distress and to indicate the time for
clinical intervention 1. The normal range of foetal heart rate is 120-160bpm.The
foetal heart rate helps in assessing various conditions: hypoxia, asphyxia,
foetal bradycardia(<110bpm) and foetal tachycardia (>160 bpm). Apart from
directly listening to the foetal heart sounds using stethoscope or pinard cone,
various devices are currently in use for monitoring foetal heart rate: foetal
Doppler, foetal electrocardiography(fECG), foetal magnetocardiography(fMC) and
foetal phonocardiography(fPCG)2.The foetal cardiotocography(CTG) which is
used in Non-Stress Test includes two transducers: one for measuring the foetal
heart rate and the other one for uterine activity. In CTG machine, the foetal
heart rate is measured using Doppler ultrasound. It is the most widely used
method to monitor the foetal heart rate. Foetal Doppler probe exposes foetus to
Ultrasound waves and shift in frequency of the reflected signal is used to
detect the heart motion. The fECG which works on by recording the electrical
activity of the heart is of two types: Direct and Indirect. In the direct
method, a foetal scalp electrode is placed directly on the foetus(in-utero), though it gives accurate
measurement of FHR it is invasive. The indirect method is a non-invasive foetal
monitoring technique obtained by placing the electrodes on the maternal
abdomen. The indirect method can be used from the 16th week of
pregnancy but the recorded fECG contains various artefacts along with maternal
electrocardiogram, therefore various advanced signal processing techniques must
be applied to obtain fECG 3. The amplitude of the fECG also decreases in
28-32 week of pregnancy due to vernix caseosa surrounding the foetus 4.
Foetal Magenetocardiogram is the recording of the magnetic field produced by
the electrical activity of the heart. It is recorded by squid array placed over
maternal abdomen and measuring foetal and maternal magentocadiographic signals
5. The fMCG is extracted and the heart rate is calculated. The major
disadvantage of this method includes: cost, trained staff required, no long
term monitoring.  In fPCG, natural
acoustic signals from the maternal abdomen are acquired using electronic
stethoscope to assess the foetal wellbeing. Though this approach is
non-invasive and requires simpler instrumentation, the foetal heart rate
measurement in this approach has two problems: firstly, the intensity of the
foetal heart sound is low compared to the interference signals such as
digestive sounds or the signals from the environment. Secondly, the intensity
and the frequency varies depending on the position of the foetus (occiput
anterior position/occiput posterior position) 6.


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