No it is not safe. You are much better off role-playing with a fake belly. The uterus is designed to stretch, but only under appropriate conditions with the proper mix of hormones to allow the stretching. You could simply rupture the organ if using a bladder, or cause an embolism if not. The body was not designed to hold air under pressure in the uterus in quantities sufficient to cause visible changes to the stomach. There are porn stars that have done inflatable plugs (usually in the anus) but these are far and few and are essentially stunt actors more than porn stars. Otherwise it could be bad, mkay?