Sorry but this story sounds really fake since the beginning. If Meghan is so hostile to you and revealed she’s pregnant with your fiancés baby, why would she lie about insemination and not just straight up tell you you they slept together. 

I hope this story is fake for your own sake too because it sounds like a really toxic relationship. But in case it is not, good thing you’re out now rather than after you’re married. 

Good luck.