They don't do it prior to 27 weeks unless there is a concern to do it! I guess it can read wrong early on. Good luck I did mine at 29 weeks and passed just don't eat anything day of! Even if they tell you to at the dr office! Do not do it! Otherwise chances are you will have to go back for the 3 hour test which is no fun!!! Even crackers turn to sugar and will give false info!
I had mine done at a little over 28 weeks. I ate a eggs and toast for breakfast about 2 hours before the test and mine was fine. My doctor's office said to eat like normal. As PP mentioned, the test is to verify your body breaks down sugar as it should and is not hanging out in your blood stream, so fasting doesn't make sense if you want an accurate result.
My theory for the reason behind a third trimester test is because that's when the baby does the most growing. But don't quote me on that.