At what point in pregnancy is a fetus considered especially vulnerable to teratogens?

The first trimester is the period during pregnancy when the embryo and early fetus are particularly vulnerable to teratogens, which are substances that can cause developmental abnormalities. During this time, the foundations for various organ systems are established, and major developmental processes occur. This stage covers the first 12 weeks of gestation and is critical because many structures are forming, including the heart, brain, and spinal cord.

Exposure to teratogens during this period can lead to significant birth defects and developmental issues due to the rapid cell division and differentiation happening as the fetus develops. For example, substances like alcohol, certain medications, and viral infections can have particularly harmful effects if encountered during the early weeks of pregnancy when the fetus is still developing.

In contrast, while teratogenic effects can occur at any point during pregnancy, the risk diminishes after the first trimester as the major organ systems are established, although the third trimester can have concerns related to growth and development rather than congenital malformations. Therefore, recognizing that the first trimester is the most critical time for teratogenic exposure is fundamental for prenatal care and maternal health education.