This is a tough place to be in. One's husband fathering a child with a girlfriend is always a possibility in poly tangles involving people of child-bearing age. While it isn't anyone's place to tell any woman what she can do with her body, it is only logical that it's a subject which should be discussed among all three of you and not among just two of you in secret. Ultimately, trying to get pregnant and having the baby is her choice, and you can't really forbid her not to, but you all need to discuss how it will affect the three of you as a family and household, legally and financially, and what each of you are ready for.
I take it you are all co-primaries? There are poly families who work these types of things out, with several children in the poly tangle sharing some parents and not others. It can be a very happy arrangement, especially if they view themselves not as a couple plus one, but a truly equal triad with the goal of living and raising children communally. You need to keep examining your fears and anxiety about this, get down to the nitty gritty of it, and keep talking, talking, talking.
The confusing thing to me is that she has contradicted herself by saying she wants to wait and yet -- they keep planning their dates around her fertility????
So, how well do you really know her, and her family and friends? I agree with Annabel about how incredibly fast you all are moving! Moving a gf in before you've spent a year together really seems very hasty and somewhat reckless.