Radiation effects depends on the amount of radiation received, the length of time exposed to it and the part of the body exposed. Some parts of the body are more sensitive to radiation than others.
It would take very high levels of radiation such as those emitted from an atomic bomb before effects from exposure would occur right away. Acute radiation sickness from very large doses of radiation, such as 100+ Rems, occurs soon after exposure. Thermonuclear war is an example of such an instance when prompt radiation sickness would occur.
Prompt radiation sickness is not a concern because radiation levels are far too small in CT for something like that to occur from one CT scan. There are regulatory limits that are set and only those that would far exceed those limits can bring about prompt effects of radiation sickness.
There is great concern for both health care workers and patients who are exposed to radiation while they are pregnant. Risks could include childhood cancer, mental retardation and abnormally small head size. No effects have occurred when exposure is within regulatory limits.
The genetic effect of radiation to parents prior to conception and to pregnant women has been studied. Results of radiation exposure prior to conception are inconclusive.
No increase in genetic abnormalities was seen in survivors of Hiroshima and Nagasaki.
Results of radiation exposure to a fetus depend on gestational age, radiation dose and location. (Occupational Safety and Health Administration)