I don't mean to burst the bubble, but just to give you my two pence, sometimes people's feelings change and it doesn't mean that something is up with them, it just means that they've changed how they feel about their partner. We all do ridiculously cute stuff in the beginning. That in itself holds no meaning anymore when the relationship ends. I remember when my ex left, crying to my best friend and saying that it didn't make any sense, how could be write what he wrote to me in these love letters he used to send all the time, then just get up and leave me a few months later? She laughed and said she had some plastic flowers in her bathroom that her ex had given her, promising that the day the died would be the day he'd stop loving her. She was right, those things mean nothing once it's over. They're just painful to cling onto.
Even if someone was up with her, for argument's sake, would that make this any better? Can you be with a woman who is so unstable she is willing to walk away from the relationship when there's a baby due? If she came back, if you chose her over your family's disapproval, could you ever shake the gut dread that she would do it again someday soon anyway? She doesn't have a good track record, to have done this more than once in such a short relationship.
My advice would be to right now, take a week or two to yourself to calm down, and then start talking about what's gonna happen when your son is born. Him coming into a world where both of his parents are calm and stable is more important right now than trying to fix things with his Mother. He's coming whether you're ready or not, you need to be as prepared as possible. Maybe your tolerance levels are different but there's no way in hell I'd allow myself to be disrespected to the extent that I was letting someone back into my life and my bed who was already done with me to such an extent that he would leave and write me a 'dear john' even while we had a baby due.
It takes some time to see things clearly and realise that you deserve better and that some people just won't make a good partner to you, whether their issues are down to psychological problems from the past or they're just them falling out of love with you. Is she acting like a good partner right now? No. She didn't come to you to try fix and work on things, she left. You deserve better. And I know she's pregnant but this doesn't sound like some little spat, it sounds like she's left you dude.