I think that you are referring to what is said about Soda and Sandy towards the very end of the book. Soda is said to be all depressed because Sandy has left him and left Tulsa and headed to Florida. That much is stated clearly.
What is implied is that Sandy is pregnant and the baby is not Soda's. I infer that because it says in the book
I guess she didn't love him like he thought she did, because it wasn't him.
I can't be sure, but I think this means that she is pregnant but is was not him that got her pregnant.
In my copy of the book, this is on page 175 out of 180 total pages -- you can look it up and see if you agree.
Maybe back then that if a girl got pregnant that she would say she is leaving to move in with her grandma, but she is going to a place where girls who are not married go and then the baby is taken away and she won't ever get to see it. Unless she finds the baby in the future, but Sandy left because she is too young to be a mom and her parents wouldn't let her get married to Sodapop.