What period of pregnancy is referred to as the time when major organogenesis occurs, risking congenital abnormalities if radiation exposure is high?

The time referred to as the crucial period for major organogenesis typically occurs between weeks 2 through 10 of pregnancy. This phase is vital because it encompasses the development of essential organs and systems in the embryo and fetus. During this time, the cells are rapidly dividing and differentiating, making them particularly sensitive to harmful influences, including radiation. High levels of radiation exposure during this period can lead to significant risks of congenital abnormalities, as it can disrupt the normal growth and development processes.

Weeks 1 through 3 are primarily focused on implantation and do not yet involve full organ development, making the embryo somewhat less sensitive to radiation effects. In contrast, by weeks 11 through 15 and 16 through 20, many major organs are already formed, leading to a lower risk of malformations from radiation. Although the fetus can still be affected during these later weeks, the initial organogenesis stage is the most critical for the potential development of abnormalities due to radiation exposure. Hence, the correct answer emphasizes the importance of this early gestational period in the context of minimizing radiation risks.