How do teratogens affect embryonic and fetal development?

Teratogens are environmental agents—including certain chemicals, drugs, infections, and radiation—that cause structural or functional abnormalities in a developing embryo or fetus. Exposure to these agents can lead to prenatal growth restriction, miscarriage, fetal death, or congenital disorders.123

The impact of a teratogen depends heavily on the timing of exposure during gestation:3

• Preimplantation: Toxic exposure may kill cells in the blastocyst, often resulting in the death of the embryo.3
• Embryonic Stage (Postimplantation through 8 weeks): This period is highly sensitive because it encompasses organogenesis, the critical window when specific organ systems are formed. Exposure during this time can cause cell death in developing tissues, leading to specific malformations. Because organ development is a tightly programmed sequence, exposure during a specific organ's "critical period" is likely to cause defects in that system.3
• Fetal Stage (9 weeks to birth): Chemical injury during this period typically results in growth retardation or functional impairments, such as learning disabilities or central nervous system damage.13

Teratogens may act directly by killing embryonic cells or indirectly by causing maternal toxicity, which deprives the embryo of essential oxygen or nutrients.3