Teratogens: Agents That Cause Birth Defects - Types, Mechanisms & Prevention

A teratogen is any agent (physical, chemical, biological) that disrupts embryonic/fetal development, causing structural/functional abnormalities.

Major Teratogen Categories

Category	Examples	Key Effects
Drugs/Medications	Thalidomide, isotretinoin (Accutane), valproate, methotrexate, ACE inhibitors, warfarin, alcohol	Limb defects, neural tube defects, craniofacial, cardiac, growth restriction
Infections (TORCH)	Toxoplasma, Rubella, CMV, HSV, Syphilis, Zika, Varicella, Parvovirus B19	Microcephaly, hearing/vision loss, IUGR, hydrops, neurodevelopmental delay
Chemicals/Environmental	Lead, mercury, pesticides, solvents, radiation (ionizing), air pollution	Neurodevelopmental deficits, growth restriction, miscarriage
Maternal Conditions	Uncontrolled diabetes (↑ cardiac, NTD), PKU (microcephaly), thyroid disease, obesity	Structural/functional anomalies via metabolic dysregulation
Lifestyle	Alcohol (FASD), tobacco (IUGR, preterm, cleft), illicit drugs (cocaine, opioids)	Growth restriction, neurodevelopmental, withdrawal syndromes

Critical Principles

• Timing: Organogenesis (weeks 3–8) = highest susceptibility.
• Dose: Threshold effect; higher exposure → greater risk.
• Genetic susceptibility: Maternal/fetal metabolism modifies impact.

Prevention

• Preconception counseling: medication review, chronic disease optimization.
• Folic acid 400–800 mcg daily ≥1 month before conception.
• Avoid alcohol, tobacco, illicit drugs.
• Occupational hazard assessment.
• Vaccinate pre-pregnancy (MMR, Varicella).

Most birth defects have multifactorial causes. Teratogen avoidance reduces but doesn't eliminate risk.