Noni big hugs

Fundal height is an estimate. If your fundal height is measuring equal to dates and the same care provider is measuring it each time rest easy. Fundal height is not an accurate science! It's a general way of checking on things. My fundal height right now is 20 weeks and I am only 17 and a bit weeks gestation. Yes, my baby is a bit bigger and I have had numerous pregnancies. But still. Don't get hung up on it...

It's fairly standard to have an u/s later on when your have GDD. This is to check that your baby is not too big. However, this annoys the living dylights out of me as the u/s weight estimates are notoriously innacurate too.

As Sherie so accurately points out having GDD does not necessarily mean a big baby. All my babies have been large except for the baby I birthed with GDD! She was my smallest.
Your baby is growing your diabetes is maintained. Try and deflect all the fear away. You are doing a wonderful job of growing your baby!