Drug use in Pregnancy- FDA Categories

In view of such events as the thalidomide crisis in the 1960’s when in-utero exposure lead to Phocomelia, and the teratogenic effects of diethylstilbestrol in 1979 US Food and Drug Administration developed strict guidelines regarding drug labeling, use of medications in pregnancy with safety parameters before it was marketed.
FDA classifies various drugs used in pregnancy into five categories, categories A, B, C, D and X. Category A is considered the safest while category X is absolutely contraindicated in pregnancy.

Pregnancy Category A
Adequate and well-controlled human studies have failed to demonstrate a risk to the fetus in the first trimester of pregnancy (and there is no evidence of risk in later trimesters).

Pregnancy Category B
Animal reproduction studies have failed to demonstrate a risk to the fetus and there are no adequate and well-controlled studies in pregnant women OR Animal studies have shown an adverse effect, but adequate and well-controlled studies in pregnant women have failed to demonstrate a risk to the fetus in any trimester.
Pregnancy Category C
Animal reproduction studies have shown an adverse effect on the fetus and there are no adequate and well-controlled studies in humans, but potential benefits may warrant use of the drug in pregnant women despite potential risks.
Pregnancy Category D
There is positive evidence of human fetal risk based on adverse reaction data from investigational or marketing experience or studies in humans, but potential benefits may warrant use of the drug in pregnant women despite potential risks.
Pregnancy Category X
Studies in animals or humans have demonstrated fetal abnormalities and/or there is positive evidence of human fetal risk based on adverse reaction data from investigational or marketing experience, and the risks involved in use of the drug in pregnant women clearly outweigh potential benefits.
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