In VERYYYYY rare circumstances. If you have a strange shaped uterus and the baby grows up into your body and it's a small baby, you can probably go quite a while without knowing. I knew this one chick who was super fit, and was at the beach wearing a bikini and fucking though she was having a heart attack because she felt something like heavy beating on her diaphragm and heart. Goes to the ER, 7 months prego. She ended up having a c-section early because the baby was compressing her organs and was causing health issues.