Gestational Diabetes

I don't get it. Can someone please explain to me why they test for this if the concern is large babies when they're going to do bi-weekly ultrasounds where they monitor their sizes anyways? My diet is top notch and I don't eat sugar or processed foods. Only some fruit. So is there some other concern?

Why not just wait till an ultrasound shows the babies are growing faster than normal indicating an issue and then do the GD test? Can't the mama go on drugs then if it should need to be controlled?