Imaging is a very important adjunct in the diagnosis of both acute and chronic diseases. There is ongoing debate as to whether imaging should be performed during pregnancy and lactation and when to discontinue imaging. This often leads to women in this period unnecessarily avoiding useful imaging exams or discontinuing breastfeeding. Obstetricians and gynecologists or other health care practitioners need to weigh the effects of radiation and contrast against the risk of not having an exam and worsening disease before deciding whether women who are pregnant and breastfeeding should undergo a particular imaging test, and to communicate with their radiologists to adjust the radiation dose to reduce the potential harm from radiation. The American College of Obstetricians and Gynecologists makes the following recommendations for diagnostic imaging during pregnancy and lactation: 1. Ultrasound and MRI are less likely to image women during pregnancy, and both should usually be the imaging of choice for patients during that period. Even so, ultrasound and MRI examinations need to be used with caution unless they can explain a clinically relevant problem or benefit the patient; 2. During pregnancy, the least desirable applications are x-ray, CT, or nuclear medicine imaging techniques that would expose the pregnant woman to radiation, although the amount of radiation caused by these examinations may be far less than the dose that would harm the fetus. If necessary, physicians should be truthful about these conditions to their patients during pregnancy, except for diseases for which ultrasound and MRI can obtain results; 3. Radioactive contrast agents should be avoided if possible. They should only be considered when the benefits of their use far outweigh the possible damage to the fetus; 4. It is not necessary to discontinue breastfeeding after exposure to a gadolinium test. I. Ultrasound in pregnancy and lactation Ultrasound should be used when there is a clinical need to expose the fetus to the lowest dose of radiation (see ALARA principles). Ultrasound uses sound waves rather than ionizing radiation. The FDA has proposed a peak limit of 720 mW/cm2 , within which the fetal body temperature is theoretically elevated by 2°C, but this is largely unlikely to be achieved. Ultrasound of all types causes the lowest fetal temperature elevation, with color and Doppler ultrasound being higher. Different ultrasound machines are configured differently, and although the risk to the fetus from the use of ultrasound is low, it should be used with caution and only when the ultrasound can explain a clinically relevant problem or provide far greater benefit to the patient than the risk to the fetus or the pregnancy. MRI in pregnancy and lactation MRI is an imaging technique that uses the signals generated by the collection of magnetic resonance phenomena to reconstruct images. Compared with ultrasound and CT, it has no radiation, no bony artifacts, multifaceted and multiparametric imaging, and a high degree of soft tissue resolution. There are no specific contraindications to MRI in pregnant women. MRI is similar to ultrasound in the diagnosis of acute appendicitis, but it is preferred because of its lower contrast rate. although there are theories that MRI has teratogenic effects on the fetus, there is no actual evidence to confirm this risk in either humans or animals. Prenatal MRI has been shown to have no effect on the fetus, and the American College of Radiology recommends MRI for women in early pregnancy. Unlike CT, MRI exams do not require contrast in most cases, and in most cases, the information obtained from MRI plain scan is sufficient for diagnosis. The two common contrast agents used in MRI at this stage are cis-iron oxide and gadolinium. There are many controversies regarding the use of the contrast agent gadolinium in the examination of pregnant women: gadolinium is water-soluble, has the potential to cross the placenta into the fetal circulation and amniotic fluid, and its use in women during pregnancy should be limited when the benefits of its use outweigh the risks. In animal studies, gadolinium has been shown to be teratogenic at high doses and repeated doses. Presumably, this is why isolated gadolinium was allowed. In studies in humans, the duration of fetal exposure to gadolinium agents cannot be determined, and no adverse effects have been reported from the use of gadolinium in women during pregnancy, but its use is still controlled. To date, no one has studied the safety of paramagnetic iron oxide contrast agents in human or animal embryos, and there is no information on their use during pregnancy or lactation. Therefore, if a contrast agent must be used gadolinium is still recommended. Water-soluble gadolinium excretion into breast milk is limited, with less than 0.04% of the daily gadolinium in the blood vessels being excreted into the breast milk. The fetus will only absorb less than 1% of the gadolinium from the mother. Although theoretically the secretion of gadolinium into breast milk can cause harm to the fetus, there are no real cases reported, so breastfeeding does not need to be interrupted after a gadolinium test. III. X-rays in pregnancy and lactation screening Women exposed to X-rays during pregnancy or before pregnancy expose the fetus to radiation greater than 1 mGy background, potentially causing significant harm. Usually, the measure of whether or not to use X-rays during pregnancy is the risk of fetal exposure to radiation, which is closely related to the gestational week of the fetus and the radiation dose. If the exposure dose is extremely high (greater than 1 Gy) and potentially fatal to the fetus during early fetal development, such doses are usually not used for diagnostic imaging. Developmental delay, microcephaly, and mental retardation are the most common adverse effects of exposure to high doses of radiation (Table 1) . The central nervous system is most susceptible to mental retardation when exposed to radiation at 8 to 15 weeks of gestation. If a radiation dose of 60-310 mGy is required to produce such adverse effects, the lowest clinically documented measure of intellectual disability is 610 mGy. Radiation exposure to this extent is rare, even when multiple x-rays are performed. No fetal growth abnormalities or miscarriages have been reported during x-rays less than 50 mGy. In the rare event that an examination above this level is required, the patient should be informed of the associated problems that may arise. The carcinogenic risk of uterine exposure to X-rays is not known, but is certainly very small. Fetal exposure to doses of 10-20 mGy increases the risk of leukemia by 1.5 to 2 points on a background of nearly 1/3000, and mere exposure to diagnostic x-ray radiation should not terminate a pregnancy. So pregnancy termination should not be based on a single exposure to diagnostic radiation alone. When a pregnant woman requires multiple examinations, a radiologist should be consulted to calculate the total possible exposure dose to the fetus in order to guide the diagnosis. Fourth, the application of CT in pregnancy and breastfeeding examination CT examination principle is that X-rays will tomography through the body, through computer calculation post-processing for secondary imaging. Post-processing can show more information, but the radiation dose of the examination is usually higher than that of a single radiography. It plays an important role in pregnancy diagnosis, and the use of CT increased by 25% per year between 1997 and 2006. the benefits and risks of using contrast in CT examinations need to be discussed. The benefits outweigh the possible risks to the fetus in the accurate early diagnosis of acute conditions such as appendicitis and small bowel obstruction. If both MRI and CT can diagnose the same disease, MRI should be preferred because she is safer for pregnant women. Radiation for pelvimetry using CT can reach 50 mGy, but can be reduced to 2.5 mGy using low exposure techniques suitable for diagnosis. in cases of suspected pulmonary embolism, the radiation exposure is lower with CT compared to ventilation perfusion scans. Oral contrast agents are not absorbed by the patient and do not cause actual or theoretical harm. Intravenous contrast agents are used in the diagnosis of soft tissue and vascular structures. the most commonly used contrast agent in CT is iodine contrast, which may cause a low risk of progressive adverse reactions (e.g., nausea, vomiting, flushing, injection site pain) and allergic reactions. Although iodine contrast agents can cross the placenta and enter the fetal circulation or directly into the amniotic fluid, animal studies have reported no teratogenic or mutagenic effects from their use. In addition, the theoretical concern of potential adverse effects of free iodine on the fetus has not been demonstrated. Despite the lack of known hazards, it is still generally not recommended unless a CT diagnosis must be obtained to obtain the information. Typically, a 24-hour cessation of breastfeeding is recommended for lactating women undergoing intravascular iodine contrast. However, due to the water solubility of iodine contrast, less than 1% of the iodine contrast will be secreted into the breast milk of the nursing woman and less than 1% of the contrast will be absorbed through the gastrointestinal tract of the infant. Therefore, breastfeeding can be done without interruption after the use of iodine contrast agent. V. Nuclear medicine imaging in pregnancy and lactation Pulmonary ventilation and perfusion, thyroid, bone, and kidney scans for nuclear studies have been labeled as radioisotope chemical agents. This type of imaging is used to determine physiological organ function or dysfunction and is not used to demonstrate anatomical structures. The exposure of the fetus to nuclear medicine examinations during pregnancy depends on the physical and biochemical properties of the radioisotope. Technetium is one of the most commonly used isotopes for brain, bone, kidney, and cardiovascular scans. The most commonly used in pregnancy is the ventilation-perfusion lung scan for pulmonary embolism detection. In general, this test results in exposure of the embryo or fetus to less than 5 mGy of exposure, which is a safe dose of radiation during pregnancy. The half-life of this radioisotope is 6 hours. All these facts support the exposure to isotope ^99mTc less than 5mGy during pregnancy. Not all radioisotopes are safe for use during pregnancy. Radioactive iodine (iodine 131) readily crosses the placenta, has a half-life of 8 days, and can adversely affect the fetal thyroid, especially when used after 10-12 weeks. Iodine 131 should not be used during pregnancy. If diagnostic tests of the thyroid gland are essential, the isotope ^99mTc should be chosen. Radionuclide compounds are excreted in breast milk at different concentrations and for different periods of time. The rate of elimination of the same radionuclide varies in different patients. Experts believe that breastfeeding can be continued after a lactating woman has undergone radioisotope screening.