Her Fearful Symmetry by Audrey Niffenegger
I hesitated for a long moment when I realized who the author of this book was, but I brought it home anyway. My bad. I hated the author's huge best-seller, The Time Traveler's Wife, and only finished it because much of the story took place in the Chicago neighborhood where Tom and Kristy were living at the time. Well - no surprise - I hated this one, too.
Once again, the author clumsily tackles the supernatural, but instead of a time traveler, this time she deals with two sets of twins and a g-g-g-ghost. (Pretty scary, huh?) I figured out almost immediately that one set of twins had pulled the old switcheroo. (Don't they always?) It took the entire book, however, for the author to reveal that fact.
It's hard to imagine a premise more ridiculous than someone becoming "unstuck in time" but the author manages it here with her ghost who comes back to life and has a baby with her bereaved lover. Hope you weren't planning on reading this book because I guess that would be a pretty big spoiler if you were. On the other hand, now you won't have to waste your time on it like I did.