In the first half, before the twist, Nick and Amy's conflict over having a baby plays a central part. You have Nick saying he was initially interested, but increasingly less so as his relationship with Andie grew, while Amy presents a version where she wants one, but only out of a self-aware attempt to reignite the marriage. And of course you believe Amy because, well, her story's the more extreme. It's one of the central contradictory parts of the case against Nick that totally sucks you in into the lies. In the final moments, in what could best be described as the film's climax, Amy reveals to Nick she's pregnant, scuppering his plans to reveal the truth in an interview with Ellen Abbott. This is one of Amy's sickest manipulations, bringing a whole new life into her scheme. And, unlike in the book, she's doing this without explicit knowledge of Nick's plans, showing a kind of forethought she's previously lacked. What makes this most interesting is that it's the usage of something he once loved to control him. Sure, it's not the same emotion driving the relationship, but it makes Nick's ultimate decision (to stay with her) that little more twisted. One point that, reading various comments across the web, has bemused film-only viewers is quite how Amy got pregnant when Nick, by his own admission, hasn't touched her. The explanation, which is elaborated slightly more in the book, is given in a one sentence reference to Nick's previous tests; she used his frozen sperm.
