During pregnancy, most fetal sensitivity to ionizing radiation occurs during which period?

• A. pre-implantation
• B. organogenesis
• C. fetal period
• D. post-implantation

Answer: (A)

Fetal radiosensitivity varies with pregnancy stage, and in this framing the highest sensitivity is during the pre-implantation period, the first two weeks after conception. At this early stage the embryo is only a handful of rapidly dividing cells. Radiation damage here can prevent implantation entirely or cause early embryonic death. If the embryo does implant and survives, the long-term congenital effects are less likely, because the damage had already determined viability rather than specific malformations forming later. That’s why this window is described as the period of greatest sensitivity in terms of potential fetal loss.

In contrast, organogenesis (weeks 3–8) is when malformations are more likely if exposure occurs, and the later fetal period involves other effects like growth retardation or CNS issues.