The Surrogate Mother by Freida McFadden

Abby wants a baby more than anything. But after years of failed infertility treatments and adoptions that have fallen through, it seems like motherhood is not in her future.  That is, until her personal assistant, Monica, makes a generous offer to serve as a surrogate.  Its an offer that will make all of Abby's dreams come true.

But soon, strange things start happening, and it turns our Monica isn't who she says she is. The woman now carrying Abby's child has an unspeakable secret and she will stop at nothing to get what she wants.