During which gestational period is the embryo/fetus most sensitive to radiation?

• A. A 22 weeks
• B. B 12 weeks
• C. C 6 weeks
• D. D 36 weeks

Answer: (C)

Radiation is most harmful to the developing embryo during organ formation in the early first trimester. At this stage, cells are rapidly dividing and differentiating to form organs, so DNA damage from radiation can disrupt development and lead to congenital malformations, miscarriage, or growth issues. Six weeks gestation sits right in this peak sensitivity window, when many organs are beginning to form and are most vulnerable.

By contrast, at around 12 weeks and later, much of the organ formation is already underway or completed, so the risk of major malformations from radiation decreases. In the late second and third trimesters (around 22 weeks and beyond), the fetus is more affected by growth restrictions or functional issues rather than newly arising structural malformations, making those periods less sensitive to teratogenic effects of radiation.