I wouldn't worry about it, the baby ay not have grown in that area yet or your baby may be a little short but I doubt anything us wrong since you Dr isn't worried!
thank you...dear...bt could you message me tht link...
Femur length is one of the markers they look for in Down syndrome babies. I don't know if I'm allowed to post a link here, but if you search PubMed articles there's a good one about research into femur length vs. Downs syndrome.
Their research and conclusion is that in babies with short femur length who DO have down's syndrome, there are lots of other physical markers present there also.
Since your doctor said everything else looks fine, there are no other abnormalities, sounds like you have a baby with slightly short legs. ;D
Looking at racks of jeans in departments stores, people's legs come in lots of lengths! Sounds like you have no reason to worry.
I don't think that it is enough of a difference to worry about too much but if your worried consult your doc. I'm sure they will explain it better.