The thin line between Chuck and Nate

Gossip Girl posts a picture of Serena looking at pregnancy tests, everyone thinks that she must be pregnant. Jenny, Dan, and Rufus see the news online, and Dan worries about what's the right thing to say. But the real thing is that the pregnancy test isn't for her, it's for Blair. Blair refuses to make the test because she's so happy cause her life is doing well, including that she came back with Nate. 
Serena tells Dan the truth about Blair's situation in the dinner with the Humphrey's family. Dan tells Serena that even though Chuck is a totally jerk, he deserves to know what's going on.

She confronts Blair over her dishonesty. Making a point by saying she exposed herself by getting the test for her.
Blair finally makes the test and later calls Serena to tell her that she's not pregnant.
Everyone see the news about Blair when school lets out. Nate initially believes that it must be a lie, but Jenny, who overheard Serena's conversation with Dan the previous night, tells him the truth. Nate gets into a fight with Chuck over the whole thing, then tells Blair he wants nothing to do with her. She is quickly overthrown as the queen bee, with Jenny joining the popular group. Serena asks Dan if he told Jenny anything relating to Blair, so he says he wouldn't do something like that to her because he loves her.
Blair is then shown going to see Chuck at a bar. She explains that he is the only person that wants to be around her now, and Chuck replies that he is no longer interested in her. Overcome by her recent troubles, Blair begs her mother to let her spend a semester in France. Serena goes to see Dan, and they both finally profess their love to each other. But their happiness is cut short when Serena discovers that Blair is going away. She races to see Blair before she gets on the plane, and convinces her to stay and fight for her life.