Hi mummy-to-be, do not stress yourself up with all the readings. My baby's femur length was also behind the normal range, sometimes behind by 2 weeks, sometimes behind by 4 weeks when she was inside my stomach. Furthermore, her head circumference was above normal range, which made it seems like she could be a down syndrome kid or suffer from dwarfism. Same as you, I went online to search for information regarding this and was scaring myself to death. But luckily my gynae was very re-assuring. Everytime I ask him, if there will be any problem with my baby, he say no worries, my fetal abnormalities scan does not show any abnormalities, and asked me to enjoy my pregnancy instead of worrying for nothing. True enough, I gave birth to my girl last november, and she's perfectly fine! Her limbs does not look short at all, and she grew from 47cm at birth to 54cm in just 2 weeks! I guess it's just because my stomach did not have much space for her to grow and stretch her legs. Cheers.
If you're still worried, maybe you can go to another gynae for a 2nd opinion. My gynae is Dr Adrian Woodworth at Compass Point.