Development of Portable Monitoring System for Real-Time Detection of Fetal Movement

Effective fetal monitoring is an important guarantee for fetal health and early treatment. Fetal movement is one of critical indicators of fetal monitoring, which plays an important role in fetal health. Counting the number of fetal movement by pregnant women is a traditional method for long-term monitoring. However, there are many defects in pregnant women’s feeling count, which cannot meet the accurate requirements of modern perinatal medicine. With the rapid development of biological and electronic technology, various sensors are used to probe the fetal dynamic monitoring, but not on fetal movement. This research proposes a monitoring method for fetal movement via three electrodes. Briefly: first, three electrodes are used to extract electrical signals in the abdomen of pregnant women; second, these signals are amplified and filtered; third, A/D converter with microprocessor is used to make analog digital conversion, which can be stored in the SD card under the control of the microprocessor; finally, the SD card data are processed by computer software and the fetal movement information is analyzed.


Introduction
Fetal health is highly concerned by family and society.Effective fetal monitoring is an important guarantee for improving fetal health and early treatment [1].
Clinical data show that abnormal umbilical cord and oligohydramnios lead to fetal distress and neonatal hypoxia are the main causes of perinatal fetal death, accompanied by abnormal fetal movement [2] [3].Fetal movement is a critical indicator of fetal monitoring which plays an important role in fetal health.Pre [4].Long-term monitoring for fetal movement has an important clinical significance for pregnant women, especially pregnant women with pregnancy induced hypertension [5].Because of the complexity of abdominal signals and the weakness of fetal movement signals, long-term real-time monitoring of fetal movement has always been a difficult problem in fetal monitoring.
At present, the traditional methods for fetal movement monitoring mainly compose of number counting by pregnant women and hospital no load test (NST).Because of the subjective defects, there is a large error in the method of number counting, which has no significant effect on reducing the mortality of the fetus.With the rapid development of biological and electronic technology, many researches are turned to the motion sensors, for instance the Doppler ultrasound probe.NST with Doppler ultrasonic becomes an important means of perinatal monitoring [6] [7].However, there are still some defects in Doppler ultrasound for monitoring fetal movement.First, the most common use of Doppler ultrasound at present is regular hospital examination.The method is designed to predict a week of 20 minute fetal movement and fetal heart monitoring which is prone to false positive; second, this test requires repeated ultrasonic monitoring of pregnant women for accuracy, which might induce subjects with conflict of emotion; Most important, the long-term use of Doppler ultrasound is controversial on whether the fetus may get hurt, which could be a damage to the health of fetus [8] [9].
At present, the method of counting monitoring for pregnant women is obviously unable to meet the needs of modern perinatal medicine.Although the Doppler ultrasound monitoring and fetal movement used in hospital is relatively mature, it cannot be used for long-term monitoring of domestic fetal movement monitoring.This study proposes a monitoring method for fetal movement via three electrodes.First, three electrodes are used to extract electrical signals in the abdomen of pregnant women; Second, these signals are amplified and filtered; Third, A/D converter with microprocessor is used to make analog digital conversion, which can be stored in the SD card under the control of the microprocessor; Finally, the SD card data is processed by computer software and the fetal movement information is analyzed.

Hardware
The overall design of the instrument hardware is shown in Figure 1.The instrument is mainly composed of the ECG electrode, the analog signal processing part and the digital signal processing section, in which ECG electrodes are uses for signal collection, and analog signal processing part is mainly used to amplify the signal and filter the high frequency plus direct current interference.The digital signal processing part mainly controls the modules and processing and Advances in Bioscience and Biotechnology storing the acquisition signals.The power supply unit of the instrument was composed of rechargeable lithium battery and conversion power supply module, which provides the required voltage for each module of the instrument with continuous power supply.Besides, convenient SD card was used as data storage for further analysis.

Preamplifier
The input of the preamplifier circuit is the ECG electrode and the output is band-pass filter.The ideal effect is that the signal collected by the ECG electrode is fully received and output to the band pass filter, and no external electromagnetic interference signal is received, and the signal is magnified at the same time.
As shown in Figure 2, in order to approach the ideal effect, the preamplifier designed in this study can be divided into two parts.The first part is a passive low pass filter and a limiting circuit, and the second part is a preamplifier.The AD4 chip uses LMC6082 integrated block, and its internal consists of two integrated amplifiers.The first integrated amplifier is used to build a voltage follower in which the acquisition signal of the control electrode could meet the requirements for the simultaneous control voltage for multiple working electrodes (input).Second integrated amplifiers are also used to build a voltage follower that connects the back electrode (FMGND) to the analog (AGND) circuit of the circuit to meet the needs of the human body and the circuit.The preamplifier output is followed by a low pass filter whose cut-off frequency is about 1.6 KHz.

Bandpass Filter
The bandpass filter is shown in Figure 3.The main chip is LMC6082 containing two integrated amplifiers.The circuit consists of a second-order high pass filter circuit and a second-order low pass filter circuit.The principle is the output of the second-order high pass filter circuit is output according to the high input impedance and low output impedance of the second-order high pass filter.For the input signal of the second-order low pass filter circuit, bandpass filtering is realized.AV1.5V is the bias voltage, raising the signal baseline from 0 V to 1.5 V, so that the signal will not distort after entering the digital signal processing circuit.

Programmable Amplifier
The programmable amplifier is mainly composed with PGA116 integrated block to construct the circuit diagram as shown in Figure 4, in which the circuit is integrated.In PGA116 module, the circuit diagram is simpler.The output is connected to the input of the ADC module of the microprocessor.The magnification can be changed by sending control commands through the SPI module of the microprocessor.

Digital Signal Processing Circuit
The design of digital signal processing circuit is mainly based on the STM32F103

Processing Software Design
The software design of fetal movement monitoring is to convert the collected signal into digital signal and store it in SD card for computer software processing.The data acquisition program involves the configuration of PGA, ADC, the interrupt and the ADC_SPI function.The basic idea of this design is that the configuration of PGA needs to change the magnification and channel two parameters, while the configuration of ADC is continuously sampled, the sampling frequency and channel change are completed by timing interruption, interrupting each time, reading the results of sampling and changing the channel.The PGA configuration and ADC_SPI function program are described in detail below.Besides, the data storage program writes the data into the SD card by the buffer register.The buffer register is the result of the ADC sampling in the acquisition program.The data is stored alternately with two registers.When the pointer of a register points to the last byte of the register, the data of the register is stored and the other register is in the register.

Results and Clinical Application
There was no ethical conflict in this study, all pregnant women and non-pregnant women were informed before tests which were approved by ethic committee.Therefore, it can be explained that the system could collect certain fetal movement signals with good elimination on interference.The limitation of this system could be interpreted that the manual counting of the peak is needed by now and the test results would be compared with ultrasonic measurement in the future.

Figure 1 .
Figure 1.Structure block diagram of instrument.

Figure 2 .
Figure 2. The construction of preamplifier.

Figure 5 .
Figure 5. Circuit of digital signal processing.

Figure 6 .
Figure 6.The waveform of experimental and control group in different stage.a: the waveform of control group; (b) the waveform of experimental group with fetal movement; (c) the waveform of experimental group without fetal movement.

FetalFigure 7 .
Figure 7. Fetal movement signals of different gestational weeks.The significant fetal movement signals were monitored at 15 weeks of pregnancy.With the increase of gestational weeks, the fetal movement signals increased gradually under the same experimental conditions, and the 39 weeks of waveform showed the most frequent fetal movement signals.(a) 39 gestational weeks fetal movement signal; (b) 24 gestational weeks fetal movement signal; (c) 15 gestational weeks fetal movement signal.
DOI: 10.4236/abb.2018.98026381 Advances in Bioscience and Biotechnology natal fetal movement monitoring is a traditional screening examination in perinatal period which could indicate fetal intrauterine risk factors before ultrasound examination