What are the risks of radiation to the fetus?
The risks associated with fetal radiation exposure depend on the gestational age of the fetus and the radiation dose absorbed  . During the first week after conception, there is an all-or-nothing effect for which the threshold radiation exposure is unknown in humans but ranges from 50 to 100 mGy preimplantation and 250 mGy postimplantation in animals. If the fetus survives this radiation exposure, the pregnancy will progress normally. After the first week of gestation, there is a risk of growth retardation with exposures above 200 mGy, organ malformation with exposures above 250 mGy, or severe mental retardation or decreased IQ with exposures above 100 mGy. All these effects were observed during the first trimester. The risk of childhood cancer increases by less than 1% with 100 mGy of exposure, while the risk of a birth defect increases by approximately 0.2% for the same exposure. As a reference, most imaging procedures result in substantially less than 50 mGy of absorbed dose to the fetus, which is well under the thresholds for the deterministic effects described above. A 50 mGy fetal exposure is associated with a 0.42% increase in the probability of childhood cancer and a 0.1% increase in the probability of birth defects.