18 week ultrasound shows right femur shorter than left

At our 18 week ultrasound, the right femur measured significantly shorther than than left but the doctor told us everything else looked "normal".  She was not sure what this indicated and referred us to the genetics department.  How serious is this?  Is there a chance the right femur could "catch up" with the left?  

[Sumer K Sethi, MD]

This can be Proximal femoral focal deficiency (PFFD), which  is an uncommon but complex problem.  In general, in individuals with PFFD, the proximal femur is partially absent, and the entire limb is overall shortened. The etiology of PFFD is not known exactly, but certain theories have been proposed and agents implicated.  Injury to the neural crest cells and injuries like anoxia, ischemia, irradiation, bacterial and viral infections and toxins, hormones, mechanical energy, and thermal injury.  Surgical and treatment options are better discussed in pediatric forum or orthopedics.