At 20 weeks the top of your uterus is around where your belly button is. You may just look like the top half is the baby bump because your uterus is pushing everything up. I looked like that for a while at first, and finally started looking more round.
I looked like that too for awhile. I was also bloated so my stomach was sort of adding on to the bump.
everyone is different...not everyone has the same pregnancy!
Becuase it's pushing your internal organs into different positions. What you are feeling up under your ribs is NOT your uterus. Your uterus starts under the pubic bone and continues up to just under the belly button at 19 weeks.
I had a "oddly placed" baby bump at first as well. My uterus happened to be more on the left side of my abdomen than in the center, but after a while(by 16 weeks) everything seemed normal. Everyone and how they carry their babies are totally different, feel different and look different.
And like some posters already said, a growing uterus does push your organs all around. If your really that scared, talk to your doctor about it, that should help relieve any concern you have with your baby bump!