I've been wearing an abdominal support belt, but my back pain hasn't decreased. Could I have a pinched nerve?

Possible. Back pain in pregnancy is extremely common. The added weight and the change in the curve of the spine both contribute to the back pain.
Not probable. I am assuming that you are referring to lower back pain. This is usually caused by a muscle strain or ligament sprain. It may be associated degenerative changes in the discs and/or arthritic changes of the spine. You do not describe any associated leg pain, numbness or tingling, that would indicate irritation of a nerve root. Therefor I do not believe that you have a pinched nerve. See your pcp.
Leg pain? Pinched nerves cause leg or arm pain back pain causef nu disc degeneration joint arthritis or muscle strain failur of bracing to relieve back pain is common for all types.