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.
Category X -
Contraindicated in pregnancy.
Studies in animals or humans, or investigational or post-marketing reports, have demonstrated positive evidence of fetal abnormalities or risks that clearly outweigh any possible benefit to the patient.
The above categories can be helpful for caregivers to assist them in prescribing medications (or making decisions to avoid others). Most doctors will try and limit their prescribing to categories A and B. Drugs in category C will only be prescribed after careful consideration and drugs from categories D and X are generally avoided altogether.
However, the pregnancy category system is not ideal and doctors who work in researching 'teratogens' (or drugs that may cause birth defects) are critical of the present system. They feel that the categories give the impression that 'category A' drugs are 'completely safe' rather than being 'safer' when compared to the other categories. Be aware that even category A medications should be avoided unless absolutely necessary during the first 12 weeks of pregnancy. It is also important to note that medications may be moved down or up categories, as new information comes to hand.
There are moves being made in the USA to try and look at alternative ways to help caregivers and women make informed decisions about prescribed medications. However the pregnancy category system is the best tool doctors have to work with at the moment.
Prescribed medications before pregnancy