Girls are strange beings. I can see two possible realities of the situation. Firstly, the obvious, she could actually be pregnant, and for both her sake, and for the kids, you'd best stay with her and don't be a total dog because as it is she's down enough.
Secondly, she could be faking it all just to see how serious you are. Sounds far fetched, but it happened to me (SO NOT FUNNY!).
My final point is that this girl really likes you, and it seems that until this happened, you liked her too. Why would you want to give up a relationship where she needs you that much? Its so hard to find someone who really likes you genuinely these days, will you find another again?
my two cents