I wouldn't stress.

Both of mine were measuring roughly on dates the for the whole pregnancy but at 20 weeks DS's kidneys were slightly smaller than normal. They did a precautionary U/S at 32 weeks to make sure all was ok & they said it can be quite normal to have bubbas or "bits & pieces" measuring a bit behind as they usually have a growth spurt in the following few weeks which catch everything up!

If they were concerned they would have contacted you by now. Try not to stress too much chick. Bring up your concerns at your next appt but I'm sure it'll all be fine!