|Fig. 1: Feto-30-semanas. (Source: Wikimedia Commons)|
It is well-known that radiation exposure is dangerous to human health. However, the details are not commonly known and sometimes hard to infer from the scientific literature. It is even harder to infer that how dangerous radiation exposure is to fetus (Fig.1). Here, we review what is learned about radiation damage during gestation from prenatally exposed atomic bomb survivors. 
Otake et al. studied the effects of the radiation exposure among the prenatally exposed atomic bomb survivors with various metrics: severely mentally retardation, intellectual capability, performance in school years, seizures, small head sizes, and average heights. 
Three results are particularly significant. First, higher radiation exposure leads the higher rate of disability. All metrics show that high DS 86 uterine absorbed dose (> 1.0 Gy) leads severe mental and physical damage. Second, the radiation exposure in earlier stages brings more severe effects. For the same radiation dose, embryos and fetuses at 8 - 25 weeks had more severe disorder than fetuses at 26+ weeks. Third, the metrics above are highly correlated each other. For example, the Otake et al. mentioned that there is a high correlation between the severely mentally retarded cases and seizure cases, and the intellectual capability and performance in school years. 
Before reading this review paper, the main question I had about the radiation exposure effect to fetuses was if it brings any intellectual disability. The following section explains about it in more detail.
To see the radiation exposure effect on the intellectual ability, researchers measured Intelligence Quotient (IQ) scores as a reference.  Researchers divided the examinees into four groups: children exposed by radiation at (1) 0-7 weeks, (2) 8 - 15 weeks, (3) 16 - 25 weeks, and (4) 26+ weeks after ovulation. The result shows that there is no statistical evidence of the radiation exposure effect for the group of 0-7 weeks and 26+ weeks. However, there are noticeable diminution effects on IQ for those who in the 8 - 15 weeks and 16 - 25 weeks groups. Especially, for the 8 - 15 week group, the decreasing rate of IQ was linear to the DS 86 uterine absorbed dose. The decreasing slope of IQ was 29 points/Gy of DS 86 uterine absorbed dose when the mentally retarded patients were included and 25 points/Gy of DS 86 uterine absorbed dose when the mentally retarded patients were not included.
© Inrak Choi. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 M. Otake, "Review: Radiation-Related Brain Damage and Growth Retardation Among the Prenatally Exposed Atomic Bomb Survivors," Int. J Radiat. Biol. 74, 159 (1998).
 W. J. Schull, M. Otake, and H. Yoshimaru, "Effect on Intelligence Test Score of Prenatal Exposure to Ionizing Radiation in Hiroshima and Nagasaki," Radiation Effects Research Foundation, RERF TR 3-88, 1988. [Congenit. Anom. 29, 309 (1989)].