It's possible, but the technicians usually measure a couple of times to be sure.
Gestational diabetes causes large babies. That is one of the reasons to treat it aggressively early in the pregnancy.
At this stage, there is not much you can do about it. You need to keep the sugars under control, because there are other problems that can arise. At least, now days with the ultrasound, the OB can be aware of how large the baby is before delivery and be prepared to take measures if necessary.