Everything Meg said is true, for sure. It's her call. She's the one living in her body and you need to trust her to know what she can and can't do.
Let her tell people herself! The rest of the company doesn't need to know unless she chooses for them to know. There are good reasons why many people choose to not announce pregnancy until after the first trimester. I can think of only two people who you should suggest she tell: the ASM (because they will be on deck with her during performances) and the costume designer. Again, it's not generally for you to tell. Offer to tell the ASM/designer if she wants, but she's probably going to choose to do it herself. Feel free to accompany her on that discussion if she wants moral support, but it's her news to share.
If she's 10 weeks along at opening, she probably won't be showing her pregnancy yet... but if you've got a 4 week run, she might have a little a bump by the end of it. The costume designer can give her clothes with a little room for her body to grow, or a double in a larger size.
It's not likely that she'll have any sort of medical emergency (of course we don't want that to happen with anyone!) but if something comes up, if she passes out or something, it would be helpful if you can tell medical providers/EMTs that she is pregnant.
If she has morning sickness, it would be nice of you to keep small trashcans with extra liners on either side of backstage, in the green room, etc. If she gets sick, the bag can be tied off and thrown out, and just put a new liner in the can.