How much difference can a week make? (growth question) XP from 2nd Tri

piper09

I posted here yesterday asking about short femur bones.  After a lot of googling (bad me) and thinking last night, I'm actually not freaking out TOO much.

I was charting when I got pregnant and I'm *almost* positive about my dates, and according to that, I would have been 18w3d yesterday at my anatomy scan.  This baby has been measuring ahead from the beginning, and my doctor has the gestational age at 19w2d, almost a week ahead.  Yesterday, he was measuring 19w exactly, even with the short bone measurement.

Soooo...is it possible that the femur bones are not really that short, and that the discrepancy in the dates is causing a false low measurement?  My doctor did say that we're only talking a matter of mm here, and that even one mm longer would cause the percentile to go up almost 20%.  I also read that femur bones grow in spurts and are the last bones to grow, as babies tend to grow from head to toe (not sure how true this is).

Not sure if this even makes sense...thanks for reading!  Am I grasping at straws here?