No nasal bone

I recently had a 20 week level 2 ultrasound in which the dr had concern about the abscence of a nasal bone.  I did have the nueclo- transluency test at 12 weeks in which no markers were indicated and my statistics were 1 in 10,000.
When the technician first did all the measurements, she said everything looked great.  The Dr., however, questioned her marking of a nasal bone and said it was skin.  I am caucasian, 35, and have a healthy 2 year old boy.  My husband's sister has an 11 year girl with downs.  She also has 2 other  younger children without downs.  No other comments were made about limb length, heart issues, etc.  Can this be the only marker?