Sensitivity to radiation is probably highest:

Sensitivity to radiation is highest during fetal development due to the rapid and complex processes of cell division and differentiation that are occurring at this stage. The developing fetus is particularly vulnerable to the effects of radiation exposure because the cells are rapidly dividing and differentiating into various tissues and organs. Disruption of these processes can lead to significant developmental issues, including congenital abnormalities, growth retardation, and increased risk of cancer later in life.

During this critical period, even low doses of radiation can have profound effects, as the cells are more susceptible to damage and mutations. This heightened sensitivity is a key reason why there are strict guidelines and precautions concerning radiation exposure for pregnant individuals, especially in medical settings.

In contrast, sensitivity to radiation tends to decrease as individuals grow older, as the cells have more capacity for repair and the structures of mature tissues are more stable compared to those under development. Hence, fetal development stands out as the period of highest sensitivity.