Ionizing radiation causes breaks in DNA strands and thus disturbs replication. The effects it has on the embryo depends on the absorbed dose (lethal dose: 150 cgy - centi Gray - in gonad dose), and on the developmental stage of the embryo or fetus.
In Hiroshima and Nagasaki after the nuclear irradiation one determined that especially injuries in the area of the nervous system and the eyes occurred that resulted in psychomotoric retardation, microcephalia, spina bifida cystica and ophthalmic abnormalities (cataracts).
Cerebral malformations were never diagnosed when an irradiation was below 50 cGy. According to this data, during a pregnancy, the dose of radiation, directed at the gonads, should never exceed 10 cGy.
During a radiodiagnostic examination 2 cGy are emitted. A single x-ray should not, therefore, be grounds for an abortion.
Nevertheless, in pregnant women, for safety reasons, every radiodiagnostic examination should avoid the pelvic region when possible.