The cause of the itchy belly is usually just the skin expanding. Keeping yourself and your belly well hydrated will usually help. This is where the creams and oils come in handy.
A nice moisturizing will go a long way with the itchiness. There are also special baths, like oatmeal baths designed to be safe and yet effective in taking care of itching. If it is accompanied by a rash or severe talk to your practitioner, you may have PUPPPs.