Happened to my daughter last year. She had been gaining weight and none of us even considered pregnancy as a cause because she was on birth control.
At one point, my wife finally said, "Honey, I think you need to take a pregnancy test." She did, and sure enough, she was five months along. Turns out there was a two day gap in her birth control coverage, and that's when it happened.
I think the size of the preborn at 5 months is a lot smaller than at 9 months.
Being a guy I don’t know what all the changes are in a pregnant woman but from what I’ve seen, read, and heard, there are quite a few. Perhaps some women are lucky and don’t get the cravings and mood swings that accompany pregnancy. On the other hand teen girls can have those same things so what do I know...