Quoting Chanda Smith:" They said at 20 weeks I was measuring for 24, 24 I was measuring for 28 and so on. It's been 4 weeks ... [snip!] ... haha.... I did get checked for GD at 28 weeks and I'm assuming I didnt have it cuz they never said anything to me about it.."
At your anatomy scan they would have gotten measurements that would average the gestational age of the baby, though sometimes measurements and averages are off. They thought I was a month farther along till my anatomy scan when it was confirmed that the average was what we had originally thought. Though at 16 weeks my measurements, other than baby's since I never got a dating US, were at 20 weeks. By 18 I was measuring 25 weeks. I'm now 24 weeks and five days and my measurements are still farther along though its catching up. Sometimes measurements of the mother really are off, but who knows.