When during pregnancy does the risk of birth defects from radiation exposure peak?

The risk of birth defects from radiation exposure is heightened during the first trimester of pregnancy. This period is crucial because it encompasses the earliest stages of fetal organ development and differentiation. The developing fetus is particularly sensitive to teratogenic effects during this time, as major organs begin to form. Any exposure to ionizing radiation can disrupt the normal growth and development processes, potentially leading to congenital abnormalities or miscarriage.

In contrast, the sensitivity to radiation exposure decreases in the second and third trimesters, as the major systems and structures are already developed. Additionally, while the amount of radiation exposure is an important factor, specific thresholds like "10 rad" do not define the risk across different timeframes of gestation. Moreover, the risk is not distributed equally throughout the pregnancy; it significantly diminishes in later stages as the fetus matures.