Pregnancy category

The pregnancy category of a pharmaceutical agent is an assessment of the risk of fetal injury due to the pharmaceutical, if it is used as directed by the mother during pregnancy. It does not include any risks conferred by pharmaceutical agents or their metabolites that are present in breast milk.
Every drug has specific information listed in its product literature. In the UK, while no preset categories are applied, the British National Formulary gives a table of drugs to be avoided or used with caution in pregnancy, and does so using a limited number of key phrases.

United States

In 1979, the United States Food and Drug Administration (FDA) introduced a classification of fetal risks due to pharmaceuticals. This was based on a similar system that was introduced in Sweden one year earlier.
The United States FDA has the following definitions for the pregnancy categories:

United States FDA Pharmaceutical Pregnancy Categories
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.

One characteristic of the FDA definitions of the pregnancy categories is that the FDA requires a relatively large amount of high-quality data on a pharmaceutical for it to be defined as Pregnancy Category A. As a result of this, many drugs that would be considered Pregnancy Category A in other countries are allocated to Category C by the FDA. See below.
The FDA has proposed updating its approach to labeling http://edocket.access.gpo.gov/2008/pdf/E8-11806.pdf

Australia

Australia has a slightly different pregnancy category system from the United States - notably the subdivision of Category B. The system, as outlined below, was established by the Congenital Abnormalities Sub-committee of the Australian Drug Evaluation Committee (ADEC).

ADEC Pregnancy Categories (Australia)
Pregnancy Category A	Drugs which have been taken by a large number of pregnant women and women of childbearing age without an increase in the frequency of malformations or other direct or indirect harmful effects on the fetus having been observed.
Pregnancy Category B1	Drugs which have been taken by only a limited number of pregnant women and women of childbearing age, without an increase in the frequency of malformation or other direct or indirect harmful effects on the human fetus having been observed. Studies in animals have not shown evidence of an increased occurrence of fetal damage.
Pregnancy Category B2	Drugs which have been taken by only a limited number of pregnant women and women of childbearing age, without an increase in the frequency of malformation or other direct or indirect harmful effects on the human fetus having been observed. Studies in animals are inadequate or may be lacking, but available data show no evidence of an increased occurrence of fetal damage.
Pregnancy Category B3	Drugs which have been taken by only a limited number of pregnant women and women of childbearing age, without an increase in the frequency of malformation or other direct or indirect harmful effects on the human fetus having been observed. Studies in animals have shown evidence of an increased occurrence of fetal damage, the significance of which is considered uncertain in humans.
Pregnancy Category C	Drugs which, owing to their pharmaceutical effects, have caused or may be suspected of causing, harmful effects on the human fetus or neonate without causing malformations. These effects may be reversible.
Pregnancy Category D	Drugs which have caused, are suspected to have caused or may be expected to cause, an increased incidence of human fetal malformations or irreversible damage. These drugs may also have adverse pharmacological effects.
Pregnancy Category X	Drugs that have such a high risk of causing permanent damage to the fetus that they should NOT be used in pregnancy or when there is a possibility of pregnancy.

The subcategorisation of Category B, while offering additional information which may be of benefit in evaluating the risk vs benefit, presents its own problem of data reliability - since human data is lacking or inadequate, the subcategorisation is based on animal data. Furthermore, allocation of a drug in Category B does not necessarily imply greater safety than Category C.
Drugs in Category D are not absolutely contraindicated in pregnancy, unlike Category X. In some cases Category D was assigned to a drug on the basis of suspicion.

Germany

Category	Description
Gr 1	Extensive human testing and animal studies have not shown that the drug is embryotoxic / teratogenic.
Gr 2	Extensive human testing of the drug have not shown that the drug is embryotoxic / teratogenic.
Gr 3	Extensive human testing of the drug has not shown that the drug is embryotoxic / teratogenic. However, the drug appears to be embryotoxic / teratogenic in animals.
Gr 4	There has been no adequate and well-controlled studies of the drug's effects on humans available. Animal studies have shown no embryotoxic/teratogenic effects.
Gr 5	There has been no adequate and well-controlled studies of the drug's effects on humans available.
Gr 6	There has been no adequate and well-controlled studies of the drug's effects on humans on pregnant women. Animal studies have shown embryotoxic/teratogenic effects.
Gr 7	There is a risk that the drug is embryotoxic/teratogenic to the human fetus, at least in the first trimester.
Gr 8	There is a risk that the drug is toxic to fetuses throughout the second and third trimester.
Gr 9	There is a risk that the drug causes prenatal complications or abnormalities.
Gr 10	There is a risk that the drug causes hormone specific action on the human fetus.
Gr 11	There is a known risk that the drug is a mutagen/carcinogen.

Categorization of selected agents

The data presented is for comparative and illustrative purposes only, and may have been superseded by updated data.

Classification of some agents, based on different national bodies
Pharmaceutical agent	Australia	United States
Acetaminophen/Paracetamol	A	B
Acetylsalicylic acid/Aspirin	C	D
Amoxicillin	A	B
Amoxicillin with clavulanic acid	B1	B
Cefotaxime	B1	B
Diclofenac	C	C
Isotretinoin	X	X
Leflunomide	X	X
Loperamide	B3	B
Paroxetine	C	D
Phenytoin	D	D
Rifampicin	C	C
Thalidomide	X	X
Theophylline	A	C
Tetracycline	D	D
Triamcinolone (skin)	A	C