How can alcohol and certain drugs act as teratogens?

Teratogens are agents, including alcohol and certain drugs, that cause structural or functional abnormalities in a developing embryo or fetus. These substances act through several biological mechanisms:12

• Direct Cell Damage: Some teratogens cause malformations by directly killing embryonic cells. During the postimplantation period, if these cells are actively dividing to form specific organs, the loss of those cells results in organ-specific malformations.2
• Indirect Toxicity: Teratogens can act indirectly by inducing maternal toxicity, which may deprive the embryo of essential oxygen or nutrients.2
• Disruption of Organogenesis: Because organogenesis—the formation of organs—is a tightly programmed sequence, exposure during a specific organ's "critical period" is highly likely to cause defects in that system.2
• Chemical Interference: Substances like ethyl alcohol or its byproduct acetaldehyde can interfere with normal development, leading to fetal alcohol syndrome. Similarly, drugs may cross the placental barrier to reach the fetus.34

Because even low doses can pose risks, medical consensus emphasizes that avoiding these substances during pregnancy is a vital ethical imperative for protecting fetal health.534