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
[41]. 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
[42]. 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
[43]. 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
[43][44]. Combining MRI and ultrasound to confirm the condition of talipes or lower-extremity impairment is highly associated with myelomeningocele or open spinal dysraphism
[45]. 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, it was able to clearly calculate the change in fetal lower limb movement angle in a pilot experiment
[46].
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
[47]. 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
[48]. With the improvement in the level of detection accuracy, cine-MRI began to be used to detect the whole fetal movement
[33]. 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
[49]. 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
[36]. 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
[32][36][50]. 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
[32]. 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
[50][51]. 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
[36]. A series of measurement methods can effectively improve the measurement of fetal lower limb movement.