Gestational Age
The gestational age is the most important factor in determining what kind of effect will occur.
- Before implantation has occurred, the most likely effects are loss of the embryo (dose dependent) or no effect. It seems that the cells at the preimplantation stage are capable of differentiating into any of the required cells for development to occur, and the loss of a significant number of these cells leads to demise of the embryo.
- Organogenesis occurs between implantation and the commencement of the foetal stage. During this period the precursors of all major organs are formed. Damage to these prognenitor cells may lead to severe development abnormalities (such as failure of organ formation or structual defects).
- The foetal period (from about 6 weeks after conception) is more resistant to serious abnormalities or prenatal death. Animal studies suggest a high rate of growth retardation. Human evidence suggests microcephaly, growth retardation and mental retardation are more common, particularly in the early foetal period.
Irradiation in all periods can increase the rate of childhood cancer, which is more dose dependent.
Dose
The amount of dose delivered is also important in determining the likelihood and severity of effects.
At doses under 0.1 Gy there is unlikely to be any significant effect on organogenesis or foetal development as the threshold for these problems is not reached. When doses climb above this level in the organogenesis period, it is recommended that women be councilled over the potential for congenital abnormalities induced by radiation. Depending on your religious or ethical viewpoints, it is also recommended that therapeutic abortion of the pregnancy be discussed with women who are irradiated in the organogenesis period.
The likelihood of stochastic effects is related to the exposed dose in a linear fashion.
Limits of data on humans
Human foetuses are not routinely exposed to ionising radiation for the purpose of determining effects on development. Human data must be obtained from other means, and the best source of data is from survivors of the atomic attacks on Hiroshima and Nagasaki. The remaining implications of in utero exposure are made from animal studies.
- There is no evidence in humans that radiation increases the rate of death during the implantation period. This data must be interpreted from animal studies.
- There is no compelling evidence that exposure during organogenesis leads to significant abnormalities from human studies. Data is interpreted from animal studies.
- Data on exposures after embryogenesis is limited to survivors of the atomic bombs; and is limited to evidence of microcephaly and mental retardation.
