During which period are teratogens considered most dangerous to fetal development?

The first trimester is considered the most critical period for fetal development in relation to teratogen exposure. During this time, the foundations for major organ systems are established, and the developing embryo is particularly sensitive to external factors that can disrupt normal development. The rapid cell division and differentiation occurring in the first trimester increase the vulnerability to teratogenic effects, leading to potential birth defects, developmental delays, and other serious outcomes.

In contrast, while teratogens can still affect the fetus in the second and third trimesters, the risk of major structural abnormalities due to teratogen exposure is significantly lower than in the first trimester. By the time the third trimester occurs, most of the fetal organs have already developed, and the focus is more on growth and maturation rather than formation. During labor, the risk from teratogens is negligible since the major developmental processes have already taken place. Therefore, recognizing the heightened sensitivity of the first trimester is key to understanding the implications of teratogenic exposure during pregnancy.