My doc was pretty upfront about this being an inexact science. I think the measuring is done to make sure the baby is growing at a healthy pace. When I was at 40 weeks, my doc did an ultrasound to make sure their was enough fluid in there to sustain the pregnancy. (Apparently, when you go past your due date, that is one of the concerns.) After doing his measurements he said, "I think you're going to be impressed by the amount of fluid that's in there when your water breaks." He also told me that docs have a lesser rate of accuracy guessing a baby's weight than second and third time moms.
I wound up being induced. When they ruptured my waters, it was like a flood.... then they moved me to another bed, which moved baby's head away from the canal a bit and the second "wave" came out, soaking the new bed and all it's pads. He was right.... it was very impressive. :-)
But fluid is but one of many factors that can effect the size you are measuring at. Definitely talk to your doc if you're concerned, but try not to stress until they tell you there is something to stress over. Enjoy!