Pregnancy Safety Index

United States Food and Drug Administration (US FDA) has used pregnancy safety index categories to determine the potential of a medicine to harm a pregnancy. The harm means birth defects or termination of pregnancy. FDA created this guideline in 1975 for pharmaceutical companies, and then revising the labeling method in 1997.

The categories are determined by the reliability of documentation from animal and human study; and the risk to benefit ratio of using this medicine in pregnancy (according to life-saving factor vs pregnancy factor). For your information too, these categories does not include any risks of breastfeeding-related risk.

The categories are listed here.
Category A

"Adequate and well-controlled 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)."

This is the list of medicines that are generally considered safe during pregnancy with the possibility of fetal harm being remote, having been tested in female animals and women. In both of them this medicine was proven safe.
Example of medicines in this category: pyridoxine (vitamin B6).
Category B

"Adequate, well controlled studies in pregnant women have not shown increased risk of fetal abnormalities despite adverse findings in animals, or in the absence of adequate human studies, animal studies show no fetal risk. The chance of fetal harm is remote, but remains a possibility."

No evidence of risk in humans. Includes drugs, which have shown no fetal risk in animals but have not been tested in women. Also includes drugs, which have shown an adverse effect in animals but not in women.
Example of medicines in this category: ondansetron (antiemetic), amoxicillin, amoxicillin-clavulanic acid, cefotaxime (antibiotics), paracetamol (NSAID), lansoprazole (PPI).
Category C

"Risk can not be ruled out- Adequate, well-controlled human studies are lacking, and animal studies have shown a risk to the fetus or are lacking as well. There is a chance of fetal harm if the drug is administered during pregnancy; but the potential benefits may outweigh the potential risk."

Adverse effects have been seen in animals but the drugs have not been tested on women. Also includes drugs, which have not been tested in animals or in women. These can be used only if the risk to the fetus is justified by its greater benefits.
Example of medicines in this category: diclofenac (NSAID), omeprazole (PPI), corticosteroids.
Category D

"Studies in humans, or investigational or post marketing data, have demonstrated fetal risk. Nevertheless, potential benefits from the use of the drug may outweigh the potential risk. For example, the drug may be acceptable if needed in a life threatening situation or serious disease for which safer drugs cannot be used or are ineffective."

Positive (definite) evidence of fetal risk. Their use can be justified only in life threatening situations, or a serious illness in which safer drugs have proved ineffective or unavailable.
Examples of medicines in this category: captopril (ACE inhibitor, antihypertensive), aspirin (NSAID), cotrimoxazole (antibiotic), aminoglycosides (antibiotics), anticonvulsants.
Category X

"Studies in animals or humans, or investigational or post-marketing reports, have demonstrated positive evidence of fetal abnormalities or risk which clearly outweighs any possible benefit to the patients."

These medicines are absolutely contraindicated in pregnancy. This comprises the list of drugs, which are completely contra-indicated in pregnancy since the risks far outweigh any possible benefits.
Examples of medicines in this category: isotretinoin, thalidomide, statins, contraceptive hormones, warfarin.