I didn't have the same as you, but I was told at my 20 week scan that my baby had short long bones in the arms and legs, which is a marker for acondroplasia (dwarfism). There are a number of types of acondroplasia which are quite severe and incompatible with life, BUT my MIL is very short as are a number of other family members on that side of the family (not acondroplasia, she was tested, just short) so I didn't allow myself to get too distressed until a follow up level 2 u/s was done at 22 weeks. The level 2 u/s showed the baby was bang on average, so they chalked it up to having the 1st u/s on a lesser quality machine with a less experienced technician.

Did you have the 2nd u/s somewhere different to the first one? My first was at the hospital in the radiology dept, but my second was in the MFM Unit so was quite a bit more in depth and with a tech who specifically did fetal u/s (as opposed to radiology dept tech who looked at all sorts of stuff each day).