I'm sorry for all the stress. Getting married is stressful enough (good stress sometimes, but sometimes bad stress too) without having your mother get a delusion about your fiancée on top of it.
I suggest letting her doctor know that she doesn't seem to be handling the stress of your getting married very well, and seems to be very anxious. It's possible that he can schedule an extra visit or monitor her more closely. I recommend assigning another calm family member or friend or two to sit with your mother and help her be calm during this time, as you're going to be too busy for that. I'd also suggest, that she might need to have much more quiet time on her own, than attending lots of events, so reducing the number of things she does and the number of people she has to deal with as much as possible. Being in crowds, particularly with people who she doesn't know, can be hard for people with schizophrenia. If that means she's not well enough to attend parts of your wedding (or all of it), can't meet the other parents until later, or can attend the ceremony but bow out of the reception, that's okay. Or maybe she can watch the ceremony from the gallery or near the door so she can go in and out, or have a hotel room to go to be quiet in near by. If people ask, they can be told she wasn't feeling well, and leave it at that.
Good luck and congratulations on your wedding and upcoming baby.
British Columbia Schizophrenia Society
A reason to hope. The means to cope.