Pregnancy Category C means that a medication has either shown harmful effects on animal fetuses in lab studies or has never been studied at all, and that no adequate studies have been done in pregnant women. It’s essentially a gray zone: the risk to a human pregnancy can’t be ruled out, but it also hasn’t been confirmed. The key phrase in the official definition is that “potential benefits of the drug may outweigh the risks,” which means these medications aren’t automatically off-limits during pregnancy.
If you’ve seen this label on a prescription or looked up a medication and found it listed as Category C, here’s what that actually means for you and how it fits into the bigger picture.
How the A-Through-X System Worked
The FDA used to sort all prescription drugs into five letter categories based on what was known about their safety during pregnancy. Category A was the safest, meaning well-controlled studies in pregnant women showed no risk. Category B meant animal studies showed no harm, though human studies were lacking. Category C sat in the middle. Category D meant there was proven risk to human fetuses, but the drug might still be necessary in serious situations. Category X meant the risks clearly outweighed any benefit, and the drug should never be used during pregnancy.
Category C was by far the most common classification. More than 90 percent of medications approved before 2015 landed in Category C, D, or X, with the vast majority falling into C. That happened partly because the bar for getting placed in Category C was low: if a drug hadn’t been formally studied in pregnant women (which was almost always the case, since researchers rarely conduct clinical trials on pregnant participants), it defaulted to C. Many of those drugs were likely safe but simply lacked the human data to prove it.
What “Animal Studies Showed Risk” Actually Means
A Category C classification could be triggered in two different ways. The first: animal reproduction studies found some kind of problem, like reduced fetal body weight, delayed bone development, or structural abnormalities, but no one had done equivalent studies in humans. The second: no animal studies had been done at all, and no human studies existed either. Both situations got the same letter grade, which is one reason the system drew criticism.
Animal studies typically involve giving pregnant rats or rabbits doses of a medication, sometimes at levels much higher than a human would take, and then examining the offspring. A drug might cause developmental toxicity in rats at very high doses but show no structural birth defects (teratogenicity) in rats or rabbits at normal doses. That kind of nuance got flattened into a single letter. A medication that caused minor delays in bone development in rats at triple the human dose looked the same on the label as a medication that had simply never been tested.
Why the Letter Categories Were Replaced
The FDA retired the A-through-X letter system on June 30, 2015, through the Pregnancy and Lactation Labeling Rule. The core problem was that a single letter couldn’t capture enough information. Category C, in particular, grouped together drugs with very different risk profiles, and both patients and providers sometimes treated the letters as a simple safety ranking rather than reading the actual data behind them.
Under the new system, drug labels include three detailed subsections. The Pregnancy subsection provides a risk summary, clinical considerations for use during pregnancy, and whatever human or animal data exists. The Lactation subsection covers whether the drug passes into breast milk and what effects it could have on a nursing infant. A third subsection, Females and Males of Reproductive Potential, addresses whether pregnancy testing is needed before starting the drug, contraception recommendations, and any effects on fertility.
Drugs approved after June 30, 2015, use the new format. Older drugs approved on or after June 30, 2001, are being phased into the new labeling gradually. You may still see the old letter categories on some medications, especially generics or older prescriptions, which is why understanding what Category C meant still matters.
Common Medications That Were Category C
Category C included a wide range of everyday medications. Aspirin was classified as Category C, along with several antibiotics like gentamicin and amikacin. Tuberculosis drugs including isoniazid, rifampicin, and pyrazinamide all fell into this category. Antimalarial drugs, antifungal drugs, and bronchodilators (commonly used for asthma) were also Category C. Some antidepressants carried this classification as well.
The sheer number of common medications in this category is part of why it caused so much confusion. A pregnant person with asthma, a bacterial infection, or depression could easily find that every treatment option available to them carried the same ambiguous “C” label, which made the letter essentially useless for comparing one drug against another.
How Prescribing Decisions Were Made
When a pregnant patient needed a Category C medication, the decision always came down to weighing the specific risks of the drug against the specific risks of leaving the condition untreated. An untreated infection, uncontrolled asthma, or severe depression during pregnancy carries its own dangers for both the mother and the fetus. In many cases, the risk of not treating was greater than the uncertain risk of the medication.
The first consideration was always whether a non-drug approach could work. When medication was necessary, providers looked beyond the letter category to examine the actual animal and human data available for that specific drug, the dose needed, the trimester of pregnancy, and the severity of the mother’s condition. The American College of Obstetricians and Gynecologists publishes guidance on specific drug classes, including psychiatric medications, diabetes drugs, asthma treatments, and blood pressure medications, giving providers more detailed safety assessments than a letter grade ever could.
For patients on newer medications with limited safety data, or those taking multiple drugs, a referral to a maternal-fetal medicine specialist or genetic counselor could help sort through the complex risk-benefit picture for that individual pregnancy.
What This Means If You See It on Your Medication
If you look up a medication and find it listed as Category C, the most important thing to understand is that this label does not mean the drug is known to cause birth defects. It means the information was incomplete. The drug may have caused problems in animal studies at doses that don’t reflect normal human use, or it may simply never have been formally studied in pregnancy.
The practical questions worth raising with your provider are straightforward: what are the known benefits of this medication for your specific condition, what are the known or suspected risks during pregnancy, and is there an alternative with more safety data available? For many Category C medications, years of real-world use in pregnant patients have provided reassuring safety information that wasn’t reflected in the original letter grade. Your provider can look at that fuller picture in a way the old labeling system never could.