Reading these designations without the experience of having been out in the real world of private practice, it seems like avoiding any category greater than AB' would be a fundamental principle upon which to base all pharmaceutical decisions. But I have been out in the real world of private practice, and so has your doctor. Typically, we both will use Categories A and B without hesitation. Category C, in the real world, isn't the quantum leap it seems to be. Your doctor will use Category C with little worry, even for illnesses that aren't life-threatening.
When the FDA cites these definitions, it is telling the absolute truth. It's what you can count on as the most cautious position. But your doctor will use these categories as guidelines, but then mix with them logic, medicinal track records, and anecdotal stories from his experience and that of his peers to make a conclusion about the drug as it applies to you.
Asthmatics must breathe, so a Category C drug that works better than a Category B is a better choice. Otherwise, such a patient may end up in the hospital needing Category D rescue. My point is that your doctor will weigh the risk vs. benefit of any drug and individualize his choices specifically for you. Your doctor doesn't want trouble. He doesn't want a bad baby that he caused. He surely doesn't want a lawsuit. He practices obstetrics for the love of what obstetrics is all about and would never knowingly do anything that would hurt your baby. If he makes a choice for a Category C drug, in spite of how ominous the designation is defined, he has chosen this path as the best thing for you and your pregnancy. Bottom line: your doctor uses Category C all of the time and without worry. Bad news travels fast. If there's a drug that doesn't treat a baby well, your doctor will know about it and act responsibly in avoiding it. I can't tell you how many times I've used Categories A through C in my twenty years of private practice with no problems at all.
(P.E.T.A. members, please skip this paragraph.) Studies of drug effects on animals are a valuable tool for medicines making the jump to aiding humans, but all scientists will tell you that you can't apply what happens to one species (them) to what will happen to another (us). True, abnormalities in animal studies may be a warning signal to pay attention, but modern medicine is full of protocols that use drugs harmless to us but dangerous for animals. Part of the problem is that it's hard for Moms-to-be to line up for a study to see if a particular drug will hurt their babies. So after the animal studies, this lack of evidence in humans will lump a drug into the FDA Categories that apologize for the lack of studies in humans.
Meanwhile, the biggest study of all is going on every time a pregnant patient fills a prescription. Like I said, bad news travels fast. It's hard to go wrong with your doctor's choice for even a Category C drug if it's been around for a while.
Category D can get pretty creepy, and these are used for life-threatening illness - epilepsy for example. Even so, depending on when in your pregnancy you take the medicine or how much and for how long, babies are tough, the majority doing fine even if a minority does terribly. But the mother must live or everything is lost.
I submit that it's more dangerous for your baby if you don't use a seat belt than if you use a properly supervised Category D drug. And if you smoke, well...don't get me started.
Compazine and Phenergan are both Category C. But both have been around for generations, and there are no reports of these drugs hurting babies. In cases of nausea in pregnancy, your doctor will weigh the risks versus the benefits (theoretical risks to your baby from medicine vs. risk to your baby from the dehydration of nausea), but the traditional safety of these medicines will have him reaching for these before he considers newer medicines that wear the same Category C designations but haven't been around that long.
Hyperemesis is risky to you as well as to your baby. The retching that accompanies vomiting, when severe enough, can tear your esophagus (Mallory-Weiss syndrome) or even the trachea (Boerhaave's syndrome). These are very, very rare, but I've had cases of each and they're not pretty. (P-Factor - just about 0. Really, don't expect crazy complications like these. You're more likely to see Carrot-Head accepting an Oscar.)