I actually just got back from my doctor's office with a similar question.

There are 5 risk classes of medications that are ranked according to how they affect unborn fetuses.

A - means that it has been tested on pregnant women and is been proven to be safe. Tylenol is the only one in this class.
B- means that while it has never been tested on human pregnant women (and never will), but they have extrapolated that it is safe for people based on their tests with pregnant lab animals
C- same as B, but studies found some risk with pregnant lab animals
D- I don't actually remember if there was a D
X- absolutely no way should you take this while pregnant

I was asking because I had been on Lexapro last year but stopped because I'm trying to get pregnant. However, I've been a quivering bundle of nerves lately and I thought it was worth a discussion of the risk factors. The doctor said that Lexapro is a class C drug, and that Welbutrin is the only class B anti-depressant that would work for me. However, I still don't want to take the risk, since my anxiety doesn't paralyze me or anything like that. I'll just keep exploring non-med options.

Hope that info is helpful!
LMo