I wouldn't worry if your doctors aren't. I'm also hardly showing at 30 weeks pregnant. There are a lot of things that can factor in. Where you or your partner small as babies, or are you big framed or smaller? It's near the end of pregnancy that you really start to show because that's when the baby starts to put on weight. And even if you don't it's fine.
You wouldn't be able to tell anything about prematurity by an ultrasound in any case. Like nickie says, some women just don't show much. Definitely don't worry that you not showing much means the baby is small, and also don't worry that (even if it were a small baby) it means that the baby will be premature.
Some women show at different stages in their pregnancies. I don't really start to show until about 20 weeks and I have had 2 children already... This one was the same way. It is especially so when it is your first baby or it has been a long time in between pregnancies. I didn't start to show with my 1st baby until I was about 28 weeks or so and then I packed it on in the last trimester. I would just discuss your concerns with your doctor but if your belly isn't measuring small then I doubt that they will give you another sonogram just for checking weight!