Teratogenesis is a medical term from the Greek, literally meaning monster-making, which derives from teratology, the study of the frequency, causation, and development of congenital malformations—misleadingly called birth defects. Teratogenesis has gained a more specific usage for the development of abnormal cell masses during fetal growth (see pregnancy), causing physical defects in the fetus.

There are a large number of teratogens, such as diethylstilbestrol, thalidomide and Agent Orange. The rubella (German measles) virus is also teratogenic, as is use of alcohol during pregnancy (fetal alcohol spectrum disorder) and tobacco cigarette smoking. Isotretinoin (13-cis-retinoic-acid; brand name Roaccutane), often used to treat severe acne, is such a strong teratogen whereby just a single dose taken by a pregnant woman may result in serious birth defects. Because of this effect, most countries have systems in place to ensure that it is not given to pregnant women, and that the patient is aware of how important it is to prevent pregnancy during and at least one month after treatment.

The term teratogenesis refers to the production of congenital malformations such as cleft lip and/or palate, anencephaly, or ventricular septal defect, which are medically serious abnormalities present at birth.

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