Medications in pregnancy are categorized in different groups based upon their safety.
Category “A” medications are those for which there are good studies of their use during pregnancy that show their safety to the baby in the first trimester. There are very few medications in this category.
Category “B” medications show good safety studies in pregnant animals, but there are no human studies available.
Category “C” medications may result in adverse effects on the fetus when studied in pregnant animals, but the benefits of these drugs may out weight the potential risks in humans.
Category “D” medications show clear risk to the fetus, but there may be instances in which the benefits outweigh the risks in humans.
Category “X” medications show clear evidence of birth defects in animals and/or human studies and should not be used in pregnancy.