Luckily whatever she throws at you (not literally, hopefully) there's a way to avoid an argument.
No matter what you are meant to have done, just say sorry and get over it. If you really haven't done anything, so what? Just apologise anyway. You have to stop thinking about who is right or wrong at this point. Does it matter whether or not you take the blame for a broken mug when you know it was her all along? Is your guilt or innocence a great legal thriller in the making? No. It's much easier just to say you're sorry for breaking the mug and then make her a cup of tea (if you've got any mugs left).
Say sorry again
First rule of Pregnancy Club is - apologise. Second rule of Pregnancy Club is - apologise. And if the rule book ran to 300 pages they would still all say the same thing. Say sorry again just in case she didn't hear you the first time. This time do it very calmly and mention the thing you have done/are supposed to have done/have forgotten to do while saying sorry. Not only will she hear you apologise, she'll know that you were listening to her and this should defuse her. Sneaky, eh?
Don't rise to the bait
It's much harder to have an argument if there's only one person having it. Don't walk away completely, but tell her you're going to make a hot drink and she should sit down and when you come back (with a cup for her too) you'll talk about what's bugging her. By the time you come back and hand her a hot drink she should have calmed down enough to talk to.
If you're in an argument, don't raise your voice. Keep reminding yourself that she's pregnant, and whereas she can't sometimes control her emotions, you can control yours. But whatever you do, don't say “You're only doing this because you're pregnant`'. True or not, that's a red rag to a bull to a pregnant woman.
Open your ears
If she wants to shout about a broken mug it's a good bet that's not really what's on her mind. In among the ranting and rambling there's probably something that's really bothering her. It might be nothing to do with you at all, it might be nervousness about the pregnancy, it might be something else. Who knows? Well, you won't unless you listen properly. Let her get it all out without replying or interrupting. Then when things have calmed down you can talk about it rationally.