Greenwich Park by K. Faulkner

 Helen is pregnant after several miscarriages. She's supposed to go to her first birthing class with her husband, Daniel, and her brother (Rory) and his also pregnant wife (Serena). But Helen is the only one who shows up. So she gets paired with a single pregnant woman named Rachel. And just like that Rachel inserts herself into Helen's life. At first, Helen's happy for the company because she's no longer working, her husband is at work all the time, and their house is all torn up for renovations so she doesn't really want to be there. But Rachel keeps turning up and insinuates herself into Helen's plans with other friends--then asks to move in "for a just a couple of days" which of course stretches out into weeks. Helen can't figure out what Rachel is up to. She and Daniel seem to be getting farther and farther apart. And Serena and Rory keep taking off for out of town vacations, so her friend Katie (a reporter) is really her only ally. (And thank goodness that Katie has a reporter's curiosity.)

There were a lot of interesting twists and turns in the story, and a good ending. The book has kind of an odd structure. A lot of the chapters are categorized by how many weeks pregnant Helen is. Others are flashbacks to when Helen, Daniel, Rory, and Serena were all students at Cambridge together. And then there are strange italicized chapters narrated by we don't know whom. If you liked Gone Girl, you will like this book.