My mouse is really pregnant, how do I tell if she's doing okay?

Okay, try not to worry too much. When my fancy mouse got pregnant it was very noticeable. She gave birth to 11 babies, all on her own. She did all of the work and was fine. I am not sure if there is much you could do for her if she had problems. She is likely making a nest and wants the male to stay out. Getting some hamster bed fluff is a good idea so she can further insulate the nest. My mouse often breathed heavily a few days before the birth. Your mouse is likely close now. Good call removing the male by the way, separate him from the babies until their eyes and ears are open and then try (extremely cautiously) to introduce him with just one of them; the biggest baby is the best idea. I don’t think you can do much of anything for the birth except make sure she, hate to say it, lives (mine did and she was a runt so don't worry). Watch carefully to make sure the mom takes good care of them and gets them all out, personally my mouse was horrible. Once they are older make sure they are weaned (it is a good idea to separate them from the mom for awhile once they are old enough) because sometimes they will continue to feed and the mother will not fare well. Try not to touch the babies unless she is well socialized and allows you to without a thought but be careful with them when they are pinkies! Also, move her food dish and water as close to her nest because you don’t want her to have to go too far. Take out the wheel too because my mouse use to go on the wheel with the pinkies until I removed (not a good mother as I said). Hope this helps, good luck! Feel free to respond with any other inquiries I have not covered in this ridiculously long answer, sorry! :P