1. Maternal Sensation

Maternal wearable devices can be independent of expensive laboratory equipment such as ultrasound, and can detect fetal movement over a more extended period instead of a short clinical measurement period [49]. Most accelerometer-based systems such as capacitive accelerometers and tri-axial accelerometers are not able to distinguish between fetal and maternal voluntary movements. Some study results show that startle movements and other forms of activity can be differentiated by acoustic sensor systems in association with accelerometers, effectively eliminating inconsistent noisy data resulting from maternal movements [50]. The acoustic measurement of fetal movement outcomes is not as accurate as ultraphonic scans or MRI.

Cardiotocography (CTG) is a well-developed test for detecting fetal movement. This technique is potentially essential in that compensatory ultrasound is used to identify heart rate to predict fetal hypoxia [51]. It can be used externally and internally to observe and preserve the fetal heart rate and contraction or deviation of the mother’s uterus placement. The results of the CTG test can be used to describe four features of fetal movement: fetal heart rate variability (FHRV), baseline fetal heart rate, acceleration, and deceleration. Although the CTG can measure fetal movement, the noise affecting the recordings make it less accurate; when the CTG is inclusive of accelerations, the results are practically in accordance with real-time ultrasonic imaging [52]. Intrauterine epilepsy is difficult to distinguish from the normal movement of the fetus and can only be confirmed when using ultrasound and CTG to check out the epilepsy phenomena at delivery [53]. Although the clinical effect of fetal movement reduction is not accurate and is strongly associated with stillbirth, CTG as a routine clinical method is mostly (about 80–90%) used with ultrasound to detect fetal motion reduction [54]. Recent research suggests that using CTG as a routine procedure for detecting fetal movement does not provide great advantage for diagnosis [55].

The actograph is an objective method to quantify the data from CTG, and is positively associated with antenatal ultrasound assessment [56]. It can be used to distinguish high-frequency signals such as fetal heart activity and regards low-frequency signals as fetus movement [57]. Although the subtle movements of the fetus, such as breathing are not enough to produce any signals, the vast majority of fetal movement signals can be detected by the actograph [58]. The size of the fetus has a positive correlation with fetal movement using quantified actograph output. Actograph exhibits a high false-positive rate when it is included in most CTG devices, so it is not widely used [56].

Two-dimensional sonography images were first used in the 1970s to observe the intrauterine environment of the fetus for real-time imaging [59]. Most maternal perceptions of fetal movement are usually so subjective that it is easily confused; only a few motions can be captured by the doppler device [60]. Modern brightness B-mode ultrasonography uses a curvilinear array of transducers to capture the three-dimensional image and video of the fetus in continuous motion. During an ultrasound examination, the obstetrician is required to apply the conductive gel to the appropriate examination area and record the relevant features of the fetal conditions. In order to minimize the exposure, the operator needs to monitor fetal movements for a brief period. Nevertheless, measurement of the fetal movement is generally not part of a routine prenatal ultrasonic examination. The variable speed, amplitude, direction of fetuses’ general movement disorders can be detected and analyzed by using sonographic imaging; this is an important tool for obstetricians to judge whether the fetus is healthy or not [61].

The volume of fetal growth fluid in the uterus detected by ultrasound to recognize placental insufficiency is closely related to fetal movement restriction and stillbirth [62]. Sonography is an optimum method to measure fetal movement. The only limitation to this approach isa segmentary view of the fetus’ gross movement around first 20 weeks [63]. In late gestation, Doppler ultrasound is not able to capture the whole fetus for clinicopathological examination due to the increase in fetal volume; natimortality of the fetus is still an intractable problem [57]. Therefore, the accuracy of fetuses’ motor evaluation in the later stages of pregnancy is hard to guarantee [64]. Ultrasound is still regarded as the best standard in fetal movement detection and is frequently used to produce quite clear two-dimensional images. The fetus’ general movement is usually suitable for ultrasound measurement, and is comprised of spontaneous motor behavior that is stable and sustainable from the first trimester of pregnancy and is maintained three to five months after delivery [61]. It was found that with the increase in gestational age, the frequency of all movement patterns decreased, which may be related to the uterine volume [65]. Although the measurement of fetal movements is momentous in fetal neurological measurements, with the restriction of using ultrasound during the second and third trimesters, it is not able to cover the whole movement using the standard method to measure the whole fetal movement [66]. Using ultrasound to observe fetal movement and intervention has reduced the fetal mortality rate; thus, it can also be inferred that using ultrasound and intervention therapy can effectively reduce fetal mortality [67].

The development of dynamic cine-MRI sequences in the field of MRI has led to improved techniques for detecting fetal movement during recent years. Cine-MRI scans are a novel and valuable technique that can directly observe the entire fetal movement using MRI technology [68]. The rendered image of cine-MRI can precisely capture the fetus’ gross body movement patterns and monitor the fetus in detail, especially in the equivalent period of delivery of the fetus [64]. Cine-MRI can provide more comprehensive imageology information of the intrauterine environment. Through the use of cine-MRI, it was found that changes in fetal motor behavior were positively related to the uterine cavity volume during pregnancy [33,69]. Cine-MRI can be used to identify premature and neonatal outcomes by distinguishing regular and unusual images of fetal movement [64]. Cinematography is considered the most suitable imageology method for fetal motion since its balanced steady-state free alignment makes the image more detailed and precise [70]. The cine-MRI has a lower absorption rate, and a higher signal-to-noise ratio compared to other alternative imaging methods under the same conditions. Moreover, it can also clearly display fluid and tissue boundaries, rendering images with strong contrast characteristics [68,70]. At around 28 gestational weeks, the fetus begins to have limited movement in the womb; cine-MRI can more easily distinguish changes in fetal position, as well as some bending and stretching, than other measuring methods. The fetus may occupy approximately 90% uterine content at near term, which decreases the movement of the fetus because the volume of the fetus in the uterine cavity increases [66].

Inborn brain impairments increase the risk of neurodevelopmental abnormalities after birth [71]. Evaluation of the pattern and frequency of movement during pregnancy is an efficacious method for neurological screening of fetuses with congenital brain injury [71]. Biomechanical testing combined with the cine-MRI and musculoskeletal modeling methods attempt to characterize fetal movement quantitatively. Finite element analysis and musculoskeletal measurement can quantify the kick forces and associated muscle forces, as well as fetal bone stress and strains [35,72,73]. The lower limb muscle forces generated by kicking at a certain gestational age can be quantified using the finite element analysis and adult musculoskeletal modeling method. Quantifying the strike alteration in the kicking force and muscle force produced by a simple stretching during pregnancy reveals that the stresses and strain stimuli of the fetus show an increasing trend in the second half of pregnancy [33]. At about 35 weeks, the uterine wall is much less deformed than in the first trimester; the fetal bone stresses and strains basically remain unchanged [35]. When the stress and strain stimulation of fetal lower limbs were significantly higher, the intramuscular force, particularly the kicking force, increased. In the third trimester, due to the uterine environment and constrained fetal position, the fetal kicking range decreases while kicking force increases [35]. The growth and development of skeleton is basically driven by cells; through the biomechanical stimulation of fetal tissue, such as stress and strain, the bone gradually mineralizes and forms [74]. With the development of tissue engineering, the existing research is able to simulate the production of engineered skeleton and gristle tissue by studying the natural developmental processes of chondrogenesis and endochondral ossification [75]. The calculation of stress and strain in lower limb bones has also made a great contribution to biomaterials.

Fetal movement can be effectively inferred through monitoring fetal lower limb movement to assess the fetus’ health. Numerous studies have investigated the relationship between fetal lower limb movements and healthy fetus development. The lower limb movement of the fetus and the whole-body movement of the fetus were firstly distinguished sonographically. The measures included strong movements like fetal trunk movements with kicking and weak movements such as isolated limb motion [28]. The data from 2D ultrasound and 3D ultrasound were used to observe the number of bones and fingers in the lower limb of the fetus through the coronal plane, sagittal plane, and transverse plane, respectively [29]. Abnormal lower limb movements such as limb reduction defects are generally associated with a number of diseases such as skeletal dysplasia or neuromuscular disorders, or neural tube defects. Ultrasound examination of neural tube defects shows the abnormal fetal lower limb starts at 10 weeks onwards, and as a result, isolated lower limb motion occurs [32]. Ultrasound proved to be an effective way to detect deformities and abnormalities in the lower extremities. Compared with MRI detection, real-time ultrasound examination has a certain timeliness because ultrasound can hardly obtain the full body image of the fetus in the third trimester. By comparing different ultrasound views of fetal leg movement in the late trimester, the fetal lower limb movement profile was adequately evaluated with the front of the legs facing the anterior ultrasound probe [32,34]. Combining MRI and ultrasound to confirm the condition of talipes or lower-extremity impairment is highly associated with myelomeningocele or open spinal dysraphism [36]. The Simi Motion System is a software commonly used in sports to measure the motion angles of lower limb joints. Using this software to process ultrasound imaging data, we were able to clearly calculate the change in fetal lower limb movement angle in a pilot experiment [37].

The use of MRI as an adjunct measurement to ultrasound in multiple fetal anomalies has shown beneficial effects, particularly in central nervous disorders. MRI may be valuable for differentiating the etiological heterogeneity that leads to arthrogryposis and fetal akinesia-hypokinesia deformation sequences and identifying related central nervous system abnormalities. Prolonged restriction of fetal movement in the third trimester has a favorable prognosis with the appropriate orthopedic intervention. Conversely, most fetal motor disorders due to congenital neuropathy result in an adverse pregnancy outcome due to hypoplastic lung [30]. Some neurological diseases, such as fetal akinesia deformation sequences, do not respond well to prenatal DNA diagnosis; therefore, prenatal imaging diagnosis plays a vital role in discovering and detecting these diseases [76]. With the improvement in the level of detection accuracy, cine-MRI began to be used to detect the whole fetal movement [69]. On this basis, the examination of fetal lower limbs is becoming increasingly accurate. Developmental hip dysplasia can be analyzed and calculated by cine-MR image sequences to capture the motion at the hip joint [31]. Dysplasia of the hip is the most common abnormal joint shape disease, especially when fetal breech presentation happens with fetal abnormal movement, which then affects prenatal musculoskeletal development and joint shape development [35]. Studies have used adult models and the 2D FE method to calculate fetal lower limb kicking force, and lower limb muscle exertions, stresses and strains [33,35,77]. The average uterus displacements for kicking in utero were calculated using a custom tracking software and the finite element method, then using the musculoskeletal model to predict fetal kicking of the hip joint and knee surrounding maximum muscle forces [33]. By observing the uterine wall deformation and fetal skeleton development, it was found that stress and strain stimulation increases over the second half of pregnancy [77,78]. Altered biomechanical stimulus by stress and strain in the hip joint and kick forces may reveal the link between the risk of developmental dysplasia of the hip and the specific endouterine environment [35]. A series of measurement methods can effectively improve the measurement of fetal lower limb movement.