Which part of The Outsiders did the novel mention that Sandy got pregnant?
It is interesting, but nowhere in the novel The Outsiders does S.E. Hinton say that Sandy is pregnant. Instead, Hinton suggests the idea and allows the reader to draw that conclusion himself. On page 111 of my copy, which is part of Chapter 7, Sodapop Curtis and his best friend, Steve Randle, briefly discuss Sandy and what has become of his relationship with her while Ponyboy was in hiding with Johnny Cade.
Sodapop was staring at his feet, but his ears were reddening. "...She went to live with her grandmother in Florida."
"Look," Steve said, surprisingly angry, "does he have to draw you a picture? It was either that or get married, and her parents almost hit the roof at the idea of her marryin' a sixteen-year-old kid."
Judging by the emotions expressed by Soda, Sandy's forced exile was very difficult for him. Earlier in the book, he says that he will marry her when she graduates, and within a few sentences of the discussion mentioned above, Ponyboy recalls Sandy's eyes lighting up when she looked at Sodapop.