If you are able to draw the balloon inside when you are contracting your pelvic floor muscles, then I don't think it matters that your stomach muscles are also contracting, but a genitourinary physiotherapist would know more about that. It sounds like you are doing it right. As far as progress goes with the stretching, it usually takes at least 15 practice sessions to get to 10 cm. but sometimes 20-30 sessions, and everyone is different depending on their elasticity of their tissues. If it's going slow, then try to practice twice a day instead of once a day. Ilana