Nuclear magnetic resonance, also known as “MRI,” is the use of the principle of nuclear magnetic resonance, based on the energy released in different structural environments within the material different attenuation, through the applied gradient magnetic field to detect the emitted electromagnetic waves, you can know the location and type of atomic nuclei that constitute the object, according to which the internal structure of the object can be drawn into the image. This has a large field of view, better soft tissue contrast, and is not disturbed by factors such as gas, bone, maternal body size (obesity), amniotic fluid and unsatisfactory fetal position, and can clearly show the fine structures of the central nervous system, common malformations of the chest and gastrointestinal tract, providing additional information beyond ultrasound. Indications The main indications include: excessive maternal obesity, uterine adenomyosis, low amniotic fluid, complex fetal malformations, multiple births, and fetuses already in the pelvis. It is important to note that MRI examination is not yet a substitute for prenatal ultrasound as the method of choice for fetal examination, but can provide richer information for clinical diagnosis in cases where prenatal ultrasound has been diagnosed or is unable to help. (1) cases in which prenatal ultrasound cannot provide diagnostic information due to obesity, low amniotic fluid, uterine adenomyomatosis, fetus already in the pelvis, posterior fetal cranial fossa anomalies in the second trimester and fetal position changes; (2) central nervous system anomalies that have been identified or cannot be identified by prenatal ultrasound can provide more additional information; (3) complex fetal anomalies: such as congenital cystic adenomatoid malformation of the lung, congenital diaphragmatic hernia, pulmonary isolation; (4) Evaluation of fetal abdominal anomalies; (5) Evaluation of placental anomalies and high risk; Contraindications The main ones include: pyloric atresia; pacemakers; ferromagnetic hemostatic anastomotic clips around cerebral vessels; metallic fragments of the eye’s or in any other vital organ; metallic nerve stimulators or implanted subepidermal electronic ears and miniature hearing aids. Safety Short-term exposure to the magnetic fields of MRI has not been found to be harmful to fetal development. Most scholars now believe that MRI of the fetus in the first trimester of pregnancy should be done with caution, and therefore unanimously recommend that elective MR examinations of pregnant women should be performed after the third trimester. Precautions (1) Early pregnancy (before 12 weeks): fetal MRI is prohibited; (2) Middle pregnancy (12-28 weeks): examination after 20 weeks is recommended; (3) Late pregnancy (28-40 weeks): it can be recommended. In addition, regardless of when the MRI examination is performed, the pregnant woman is not sedated and contrast agents are not used, and it is not necessary not to ask the pregnant woman to hold her breath and not to use various examination gates.