First I agree with you, it's completely awkward that she would be so open about something so insanely personal (regardless of outcome or belief). Not to mention I would think that would be grounds for some type of HR violation as that is such a sensitive subject. On top of that should could loose the respect of some staff members speaking so openly about such personal matters. I am of the honest belief that my personal life gets left at the door when I go to work. I don't even discuss my pregnancy for fear that others might be dealing with issues I am unaware that my pregnancy might trigger. I only answer questions asked if I feel ok to do so.
My recommendation would be to have a very candid talk with your boss. I would even leave the other ASM out of the conversation and focus it on your and the stores well being. I would let your boss know at this point in your pregnancy you would prefer if at all possible to have your days off break up your work days into smaller amounts so that you may continue to work until your expected leave date. With the other ASM needing time off right now you would hate to work so many days in a row and possibly go into labor early or have your doctor pull you out of work before they have the proper coverage to cover you. Again it's just a request but then should something happen you at least made your concerns known. I too feel as though work can rationalize our upcoming maternity leave as an extended vacation instead of a necessary medical leave. Therefore they expect us to buck up and just deal with the schedule they give us leading up to it. On the subject of the other ASM I would personally leave this alone as what is done is done and maybe say something if she comes back talking about it.
I'm sorry you are in this situation as it truly is uncomfortable. Best of luck!