I was wondering about uterus size... 2 appointments ago at 16 w. my doctor showed me how to feel where my uterus is, and…

I was wondering about uterus size... 2 appointments ago at 16 w. my doctor showed me how to feel where my uterus is, and at the time it was about a quarter inch below my belly button, and that buy my next appointment at 20 weeks it should be about in the middle of my belly button. I've been checking up on it almost daily to see where it sizes up, and now at 21 w. I still doesn't seem like it's grown at all and it's still under my belly button. at my U/S everything was fine with the baby but I can't help to wonder when I'll pop and if this has anything to do with it? thanks!