Hmm, I wouldn't cancel the appointment. I've had several pregnant friends with no family history of diabetes and no sign that they would have any issues either (AND had been fine in their first pregnancies!) only to test postive to gestational diabetes. A few of them had to make significant changes to their diets and lifestyle to manage it properly. Personally, it's not something I'd like to mess with. I'm yet to experience the test but I know it can be long and boring but I know I'd also prefer to know if I had diabetes.