Ionizing Radiation

Ionizing radiation in sufficient dosage has been known to disrupt prenatal development almost since the discovery of X-rays. Case reports of birth defects in children prenatally irradiated appeared in medical journals as early as 1929. The effects of high dose prenatal ionizing radiation are smaller stature, microcephaly, eye malformations, mental retardation, and lowered IQ test scores. After the atomic bombs were dropped on Hiroshima and Nagasaki in 1945, the United States and Japanese governments began cooperative research on the survivors, children who were in utero at the time of the bombings, and later-born children of survivors. Dosages of in utero ionizing radiation were reconstructed using data from atom bomb tests in Nevada in the 1950s, research with animals, and the self-reported locations of the pregnant women at the times of the bombings. Probability of severe mental retardation, IQ test scores, and teacher ratings of school performance showed dose–response relations to exposure such that higher doses yielded a higher chance of severe mental retardation, lower IQ test scores, and worse school performance. The dose–response relation for severe mental retardation was steepest for exposure at 8 to 15 weeks post-conception, a 43% increased risk, per gray, of exposure. At 16 to 25 weeks the relation was weaker, and was not detectable at less than 8 weeks or greater than 26 weeks age post-conception. Small head circumference was associated with radiation exposure 0 to 7 weeks post-conception. Seizures of unknown origin were also associated with exposure 8 weeks or more after conception. Likely mechanisms for the effects of prenatal ionizing radiation on brain development include alterations of neuronal proliferation, neuronal migration, programmed cell death (apoptosis), and synaptogenesis. Laboratory experiments show that ionizing radiation alters molecules on the surfaces of neuronal cells that contribute to cell adhesion and migration.

The effects of ionizing radiation on human health are highly controversial. Research controversies about risk estimates based on the Japanese atom bomb survivors concern whether there is a threshold below which no developmental effects of ionizing radiation  occur, whether the “nonexposed” survivors in Hiroshima and Nagasaki are an appropriate comparison group to those who were highly exposed, whether the  increase  in  voluntary  abortions  following  the bombings may have affected the results, the possibility that fetuses and embryos that were miscarried as a result of the bombings could yield a biased sample of survivors, the appropriateness of extrapolating from acute exposure in a single event to low level intermittent or chronic exposure, and the potential effects of other variables in wartime Japan, such as stress and malnutrition.

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Other developmental effects of prend postnatal ionizing radiation include an increase in childhood leukemia, thyroid cancer, other cancers, and possibly arteriosclerosis. Prior to a landmark study in England by Alice Stewart in the 1950s, diagnostic X-rays of the abdominal area during pregnancy were considered to be safe.

Experts advise that a qualified medical physicist or radiologist be consulted if a pregnant woman has received diagnostic or therapeutic ionizing radiation. It is important that the dosage be calculated accurately in order to estimate risks. Most diagnostic X-rays are estimated to have an extremely low risk to the fetus or embryo. Avoiding unnecessary diagnostic irradiation during pregnancy because of risk is advised, but is separate from the issue of choosing to terminate a pregnancy because of the very small chance of disease or malformation. For example, even if the risk of leukemia is elevated by approximately 50% over a 10-year period due to 2 rads of prenatal diagnostic ionizing radiation, elective abortions would sacrifice approximately 1,999 unaffected children in order to avoid the birth of one child who would be subject to leukemia.



  1. Brent, L. (1996). Developmental effects following radiation exposure: Counseling the pregnant and nonpregnant patient about these risks. In W. R. Hendee & F. M. Edwards (Eds.), Health effects of exposure to low-level ionizing radiation. Philadelphia: Institute of Physics.
  2. Schull, W. (1995). Effects of atomic radiation: A halfcentury from Hiroshima and Nagasaki. New York: Wiley-Liss.
  3. Stewart, M., Webb, J., Hewitt, D. A. (1958). A survey of childhood malignancies.  British  Medical  Journal,  1,1495–1508.
  4. World Health Organization, Ionizing Radiation,