First Trimester

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  • The developing CNS during the first trimester exhibits a particular sensitivity to ionizing radiation.

  • The maximum sensitivity of the human brain occurs between 8 - 15 weeks gestation.

  • This is the time of most rapid cell proliferation and migration of immature neurons.

  • During this period a 43% frequency of mental retardation occurs for 1 Gy (100 Rad) exposure with an apparent threshold in the range of 200 - 400 mGy.

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