How can radiation exposure during pregnancy cause congenital malformations?

Embryonic tissues are composed of rapidly proliferating cells, making them highly sensitive to ionizing radiation. Congenital malformations occur when radiation exposure interferes with the development of specific organs during their critical period of formation.12

The specific outcome of exposure depends heavily on the timing of the radiation:
* Early embryonic life: Exposure is more likely to result in the death of the embryo rather than a malformation.1
* Organogenesis: If exposure occurs while a specific organ is forming, that organ is likely to become malformed.12
* Later stages: Exposure is more likely to produce functional abnormalities rather than lethal effects or structural malformations.1

The risk of malformation is highest when a significant dose is received during the precise, sharply circumscribed period of an organ's development. For example, human studies following the atomic bombings of Hiroshima and Nagasaki identified the 8th to 15th week of gestation as the period of greatest susceptibility, with increased incidences of reduced head size and intellectual disability. Because the induction of these defects generally requires injury to many cells, such effects are unlikely to result from low-dose natural background radiation.12