I often hear that after receiving diagnostic radiation, such as chest x-ray, oral x-ray, or abdominal x-ray, a woman finds out that she is pregnant or has to receive x-ray during pregnancy because of a disease, so her friends and even many doctors tell her that the exposure during pregnancy will cause fetal malformations and that she should terminate the pregnancy before it is too late and induce abortion. No one wants to have an unhealthy child, so the joy and hope for the baby was immediately replaced by physical and psychological damage. A little life is killed in the embryonic stage. Is it true that the baby cannot be kept if x-ray, CT or MRI is done during pregnancy? The American College of Obstetricians and Gynecologists (ACOG), in conjunction with other relevant departments, has developed specific guidelines to standardize clinical practice and to guide the selection and interpretation of imaging examinations during pregnancy and lactation. The four editions of the ACOG guidelines are presented here to provide a better understanding and familiarity with the selection of imaging examinations during pregnancy and lactation. When pregnancy meets X-rays and CT X-rays and CT are basically the same in principle, both have ionizing radiation and are lethal and teratogenic to the fetus, but the magnitude of their effects on the fetus is mainly related to the gestational period and radiation dose at the time of examination. For example, radiation exposure at 8 to 15 weeks of gestation has the greatest effect on the central nervous system. If very high exposures (greater than 1 Gy) occur early in embryonic development, they can be fatal to the embryo. In practice, however, such high doses are not used in diagnostic imaging. The relationship between radiation-induced malformations and gestational age and radiation dose is shown in Table 1 The fetal radiation doses during common radiological examinations are shown in Table 2. Although iodine contrast agents can pass through the placenta, animal studies have shown no teratogenic or mutagenic effects. To ensure safety, it is recommended that contrast agents be used only when they can give diagnostic information to the fetus or the pregnant woman that absolutely affects treatment. Since iodine contrast agents are water soluble and are secreted in less than 1% of breast milk, there is no need to stop breastfeeding after using iodine contrast agents. Q: Can exposure before pregnancy affect the fetus? If a woman is exposed to x-rays higher than 10 rads during the first two weeks of pregnancy, it may kill the embryo. But it is a 0 or 1 problem, meaning that if the fetus survives, there will be no problem. Q: But what happens when a pregnant woman does get a chest x-ray and her baby is deformed? Remember, without exposure, the same 4-6% of newborns will have various types of malformations, but the vast majority will be minor, such as a birthmark, an extra finger or toe, etc. A child with malformations is not the result of diagnostic radiation. When pregnancy meets ultrasound and MRI Ultrasound and MRI examinations, despite their different principles, are free of ionizing radiation, have minimal impact on the fetus, and should be the first choice for imaging examinations during pregnancy. In particular, the available evidence concludes that there are no special considerations to recommend MRI in early pregnancy and that it can be used with confidence. However, it should be emphasized that the risk of MRI contrast gadolinium to the fetus has yet to be further confirmed, and it is recommended that its use be considered only when the benefits of its use clearly outweigh the risks. However, there is no need to stop breastfeeding after gadolinium-enhanced MRI examinations. When pregnancy meets nuclear medicine imaging The basic principle of nuclear medicine imaging is to use the tracer effect produced by labeled radioisotopes. The exposure of the fetus to nuclear medicine imaging during pregnancy depends on the physical and biochemical characteristics of the radioisotope. All evidence supports the safety of 99Tcm examinations at less than 5 mGy during pregnancy. However, radioactive iodine (131I) should not be used in pregnancy, either for diagnostic or therapeutic purposes. Radionuclide compounds can be secreted into breast milk at different concentrations and for different periods of time, and the amount of the same compound secreted into breast milk varies from person to person. Therefore, when they are used in breastfeeding women, it is recommended that they should be discussed with breastfeeding and nuclear medicine specialists. In summary In summary, the final recommendations given in the 2017 edition of the ACOG safety guidelines for imaging during pregnancy and lactation are fourfold: 1. Because ultrasound and MRI examinations are free of risks such as ionizing radiation, they are imaging techniques that can be chosen for women during pregnancy and lactation; however, they also need to be used with caution and recommended only when used to answer relevant clinical questions or to provide medical benefit to the patient. 2. With some exceptions, the exposure dose of radiation due to X-ray examinations, CT scans or nuclear medicine imaging examinations is much lower than the damage dose to the fetus. If these tests are necessary supplements to ultrasound or MRI or are easier to diagnose disease, then they should not be denied to women during pregnancy. 3. The use of MRI contrast agent gadolinium in pregnancy should be limited. It should only be used if it can significantly improve the diagnosis and fetal or maternal outcome. 4. There is no need to interrupt breastfeeding after the use of gadolinium agents.