Disclaimer: I am aware of the fetus/foetus argument but it seems fairly common for British books to use the 'foetus' spelling.
From a radiobiological point of view, there are three phases of in utero development.
- Pre-implantation, when fertilisation has occurred but the embryo has not yet implanted in to the uterine wall (Day 0 – 10)
- Organogenesis, the phase from implantation until the precursors of all major organs have developed (Day 10 – 42)
- The foetal period describes the remaining development in utero (Day 43 – Term)
Most studies examining the effects of radiation have been performed in mice or rats. The time estimates above are based on comparing the phases in a mouse embryo with that of a human.
Pre-Implantation
Animal studies predict that the major effect of radiation exposure in the pre-implantation period is prenatal death. This stage is the most sensitive, but if the embryo survives then it usually develops normally and few abnormalities if any are recorded.
In humans, there is minimal evidence of effects of radiation exposure at this stage. It is likely that a significant number of embryos abort or are resorbed.
Organogenesis
In animals, exposure to ionising radiation in the organogenesis phase leads to a high risk of developmental abnormality. This can include microcephaly, failure of organs to develop, intrauterine growth retardation or other gross abnormalities.
This phase is particularly sensitive as most organs are in the ‘blastula’ period of development, with minimal differentiation and heightened sensitivity to radiation. This phase is also the period when other teratogenic agents exert their effect, such as thalidomide or rubella virus.
In humans, growth retardation and microcephaly are the most frequently observed effects based on data from atomic bomb survivors. Mental retardation is also very common. Data from therapeutic or diagnostic radiation also exists, suggesting that radiation exposure may lead to increased rates of organ malformation if delivered between 4 and 11 weeks of life.
Foetal Period
In animals, radiation delivered in the foetal period leads to permanent growth retardation. Organogenesis is not as impaired and congenital abnormalities are much less common. The risk of malignancy in later life is increased as dose increases.
Human evidence demonstrates that mental retardation, growth restriction and microcephaly occur if the fetus is exposed in the foetal period. The highest risk of mental retardation occurs if exposure is between 8 and 15 weeks, although some risk continues until 25 weeks gestation.
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