around 10 weeks short femur
is something abnormal,
limb shortness can be seen in any of the three segments of it,
femur shortness can also be seen in middle segment called mesomelic dwarfism
, but don't worry these are possibilities only, sometimes some amount of shortness could be just because of things like lateral curvature of femur etc,
you follow with regular ultrasound
checkups which will help in monitoring the growth.