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Teratogens are environmental factors that have been shown to cause malformations.
Often it is difficult to establish a causal relationship between agent and malformation, because animal experiments may not give results that extrapolate to humans, and, retrospective studies are confused by the high rates of non-specific illness and multiple medications prevalent in pregnancy.
For all tetratogens, a critical period has been established beyond which there is little or no damage. Organs seem most susceptible from the fourth to the eighth weeks of gestation. However, individual susceptibility varies, and for most drugs the profile of safety in pregnancy is unknown.