It's finally time to lay the cards on the table: Nick's book is done. He shows Amy the manuscript, which is titled with an insulting, obscene epithet that's obviously pointed to her. Instead of rage and contempt, though, Amy just smiles and says she's got good news of her own. She has something to show him.
When Nick sees the positive pregnancy test, he makes her take another one in front of him. Then he drives her to the doctor and watches her get a blood test. Both point to only one conclusion: Amy is pregnant. Like, for real pregnant. Not stealing-Noelle-Hawthorne's-pee pregnant.
How did she do it if she and Nick haven't even been sleeping in the same bed? Easy. Remember the notice they got in the mail that the fertility clinic was going to destroy Nick's sperm sample and how the letter mysteriously disappeared? It didn't go in the trash. Amy saved it as another insurance policy, just in case she needed to beat Nick—and then, plainly put, she used it. Oh, and it's also a boy. Congratulations, Nick. Cigars all around.
Now that Amy is pregnant, she says she needs Nick to do a few things. First he has to delete his book, and then he has to confess that he purchased the golf clubs, porn, and television set and hid it all in the woodshed. He also has to swear that although he did once think she was framing him, he knows it wasn't true, they're in love again now, and everything is hunky-dory.
Nick knows that if he refuses, Amy will cut him off. She'll have the baby, whom Nick will never see, and her corrupt, twisted mind will be his primary influence as he grows up. He can't allow this to happen—true to his word, Nick will raise his son to be a better man than he was.
As a final act of submission, Nick calls Boney and tells her their private investigation is off. He then goes to break the news to Go that he's not leaving Amy and that she's pregnant. Go is angry and breaks down and cries. Even though Nick contends that he's motivated to be a great husband and dad and won't turn into their father, Go says he will. He and Amy are going to be a literal nuclear family—and eventually they'll detonate.