At the end of Gone Girl, Amy is pregnant and this is the reason that Nick decides to stay with her.
However, it is unclear to me who the father really is.
Is it Desi or is it Nick from before the disappearance?
Is this clarified in the book?
Nick intends to leave Amy and expose her lies, but Amy reveals she is pregnant, having artificially inseminated herself with Nick's sperm stored at a fertility clinic.
...the same is true in the novel:
Amy uses Nick's semen they had saved at a fertility clinic to make herself pregnant.