Hey sharon... This is my first pg and my GD was picked up at 14w. What I can tell you...
1. GD babies can be bigger than normal, but this is not always true.
2. If it looks like a GD baby will be too big for the mum to deliver naturally they will usually induce a few weeks early.
3. It is not standard practice for GD women to have a c/s. Most OB's only opt for a c/s if the GD is affecting mum's health or the bubs is too big and she can't deliver naturally... but due to monitoring and point 2 this isn't that common.

Please try not to worry too much until you have the test results. With diet and exercise you can control your GD and then look forward to a perfectly natural delivery.