X-rays are a form of ionizing radiation. (Ionizing radiation is the general term for energy that deprives electrons of energy-absorbing substances.) Specifically, x-rays are "high-energy electromagnetic radiation of the same form as heat and light."

X-rays are measured in units called rads. A rad is the amount of energy that is deposited in the targeted area of the body. Depending on circumstances, such as dosage and fetal age at the time of irradiation, exposure before birth may interfere with later growth and development.

To date, there is no convincing evidence that dosages below 5 rads cause fetal malformations. Dosages less than 10 rads do not significantly raise the risks of birth defects. Common procedures deliver a great deal less than these amounts.

Fragmented dosages over a period of time are unlikely to cause changes in DNA structure. However high levels at any time during pregnancy can lead to miscarriages, and cause breaks and other changes in DNA structure, including chromosomal changes.

More than any other fetal tissue, the human brain is the most likely to suffer from exposure to radiation. The most critical period is between 4 and 18 weeks after conception. Prenatal exposure to greater than 20 rads between 4 and 17 weeks can cause microcephaly and mental retardation. The National Commission for Radiation Protection recommends that cumulative fetal dose not exceed 0.5 rads.

It is important to note that x-rays affect only the areas at which they are aimed. They do not travel to parts of the body at which they are not aimed.