The stage at which the fetus is most vulnerable to teratogenic effects occurs during?

The stage during which the fetus is most vulnerable to teratogenic effects is the period of organogenesis. This phase occurs approximately from the third to the eighth week of gestation, during which the fundamental structures of the organs are formed. Because the cells are rapidly dividing and differentiating into specific tissues and organs, any harmful exposure—such as drugs, infections, or environmental toxins—can lead to significant developmental anomalies or malformations.

During organogenesis, the critical development of the heart, brain, limbs, and other organs takes place. Any disruption in this delicate process can result in congenital abnormalities. As such, this stage is particularly sensitive to teratogenic agents, making it the most critical window for preventing and understanding the impacts of teratogens.

The other options represent different stages of development. The germinal period is characterized by initial cell division and implantation, and while it is an important phase, it is less sensitive to teratogens compared to organogenesis. The fetal stage is when organ systems are developing and maturing but, by that time, most of the organ structures are already formed. Postnatal development refers to the stage after birth and is not relevant to teratogenic influences on fetal development.