Teratogenicity

Teratogenicity is the capacity of an agent or condition to cause birth defects or other developmental abnormalities in a fetus. A teratogen is any substance or factor with such potential, including certain drugs, chemicals, infections, or maternal health conditions. The field that studies these effects is teratology, and teratogens are evaluated for their risk during pregnancy.

Outcomes depend on dose, timing during pregnancy, and the fetus’s genetic susceptibility. The most vulnerable period is organogenesis, roughly the first eight weeks after conception, when major malformations are most likely to form. Exposures outside this window may cause growth restriction, functional deficits, or later neurodevelopmental problems even if no structural defect is evident.

Established examples of teratogens include thalidomide, isotretinoin, valproate and other anticonvulsants, methotrexate, warfarin, alcohol, and tobacco. Infections such as rubella, cytomegalovirus, Toxoplasma, and Zika can also cause congenital abnormalities. Environmental exposures and maternal conditions (uncontrolled diabetes, certain metabolic disorders) contribute as well.

Assessment and prevention rely on animal studies and human epidemiology, regulatory labeling, and clinical counseling. Risk varies with dose and timing and is not uniform across individuals. Prevention strategies emphasize avoidance of known teratogens when possible and optimization of maternal health, including folic acid supplementation to reduce neural tube defects.