Well, it could be a number of things. It could be normal irritation from your skin stretching to accommodate your growing uterus. But it could also be a sign of something more serious, so it's important to tell your doctor about the itchiness.
There are several skin disorders that could happen in pregnancy that can cause itchy skin — either on your belly or elsewhere — and while most aren't associated with problems in the fetus, some are. Cholestasis, a liver disorder, is one of those. It may mean an increased risk of preterm birth, fetal distress or stillbirth. So whether there's a rash or not, definitely tell your doctor, since a blood test may be needed to diagnose cholestasis.
If the doctor rules that out, think about what products you've been using on your belly. Since your skin can be more sensitive during pregnancy, you could be getting irritated by something as run-of-the-mill as a skin cream, so consider switching brands (try something made for sensitive skin!). Or if you recently started using a new soap or detergent, there's a good chance that could be what's making you feel itchy.