During which period of pregnancy is the unborn child most sensitive to ionizing radiation?

The period during which the unborn child is most sensitive to ionizing radiation is organogenesis. This stage occurs from the end of the first week after conception through the end of the eighth week. During organogenesis, the fundamental structures of the body are being developed, including vital organs and systems. Cells are rapidly dividing and differentiating, making them more susceptible to the damaging effects of radiation.

Exposure to ionizing radiation during organogenesis can result in severe developmental abnormalities, growth retardation, or even miscarriage. As the organs are forming, any disruption caused by radiation can lead to significant and often irreversible consequences for the developing fetus.

In contrast, while the pre-implantation phase is a critical time for the zygote, the risk is less about teratogenic effects and more about survival since the embryo is not yet attached to the uterine wall and is simply dividing. During the fetal period, which occurs after the eighth week, the major organs are already developed, and while they can still be affected by radiation, the sensitivity is reduced compared to the organogenesis stage. Similarly, late pregnancy would see a further reduction in sensitivity as the fetus is nearly fully developed.