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A teratogen is an environmental agent that can cause abnormalities in a developing organism resulting in either fetal death or congenital abnormality. The human fetus is separated from the mother by the placental barrier, but the barrier is imperfect and permits a number of chemical and infectious agents to pass to the fetus.

Well known teratogens include (but are not limited to) alcohol, excess vitamin A and retinoic acid, the rubella virus, and high levels of ionizing radiation. Perhaps the best known teratogenic agent is the drug thalidomide, which induced severe limb abnormalities known as phocomelia in children whose mothers took the drug.

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Science EncyclopediaScience & Philosophy: Swim bladder (air bladder) to Thallium