Overview

A teratogen is any substance, organism, physical agent or environmental exposure that can interfere with normal fetal development and increase the chance of a congenital anomaly or functional impairment. The word derives from the Ancient Greek teras ("monster"), reflecting early attempts to describe abnormal births. Teratogens act during pregnancy to alter growth, form or function rather than causing inherited genetic changes.

Common types and examples

Teratogenic agents fall into broad categories: chemical substances (industrial chemicals, heavy metals), pharmaceutical drugs, alcohol and recreational drugs, infectious organisms, and physical agents such as ionizing radiation. Classic examples that illustrate different risks include:

  • Alcohol — prenatal alcohol exposure can lead to fetal alcohol spectrum disorders, a range of growth, facial and neurodevelopmental problems.
  • Certain medications — some medicines taken in pregnancy are linked to malformations; historical events such as the thalidomide tragedy showed how a drug can cause limb defects when given in early pregnancy.
  • Infections — maternal rubella, cytomegalovirus and Zika virus have been associated with congenital abnormalities.
  • Chemicals and metals — high exposures to lead, mercury and some organic solvents can affect neurological development.

Timing and biological mechanisms

The effect of a teratogen depends strongly on dose, duration, genetic susceptibility and, crucially, timing. The embryonic period (roughly weeks 3–8 after fertilization) is when organ systems are forming and is especially sensitive to structural malformations. Later exposures may impair growth or brain function. Mechanisms include disruption of cell proliferation, differentiation, migration, induction of cell death, interference with signalling pathways, or damage to placental function.

History and public-health response

Recognition of teratogens has evolved through clinical observation and epidemiology. The mid-20th century thalidomide episodes and outbreaks of congenital rubella prompted changes in drug testing, pregnancy labelling and vaccination programs. Surveillance, reproductive toxicology testing and guidelines for prescription use in pregnancy are now standard public-health responses.

Prevention, clinical guidance and distinctions

Prevention focuses on reducing exposures, immunization against preventable infections, preconception counselling and careful medication review. Patients are advised to discuss all drugs with a clinician before conception and during pregnancy; authoritative prescribing guidance helps weigh maternal benefits against fetal risks. Not every exposure causes harm: genetic factors, nutritional status (for example, folate deficiency and neural-tube defects) and chance all influence outcomes. For more information on congenital conditions see birth defects resources, and consult professional advice before changing any regimen involving prescription drugs.