I was pregnant with DS at age 19, weighed 60kg and 171cm tall - and I had GD. I ate well and did everything right but I failed both the GCT and GTT. Although it was only by a small amount, anything over 7.7 after the GTT is considered GD. I managed it with diet obviously and my son was born at 35+5 weighing 2.87kg. Had I have gone to term, I'd say he might have been closer to the 4kg mark - no a giant baby but a healthy sized bub.

In answer to the OP's question though, yes you can refuse the test, but if you plan on birthing in a birthing centre or are considered high risk, I don't think it would be wise. Is there any particular reason you wish to refuse it?