my friend was told late in her third trimester that her baby was a dwarf because her limbs measured too short. When she asked if there was any chance they were wrong, she was told she could hope for a miracle. they prepared for the worst and had every specialist under the sun on hand to deal with complications when the baby was born. long, stressful story made short, her miracle was born perfectly normal. mom and dad are both petite, and baby is on the petite side too. But perfectly healthy and normal.
I was also told that my son had a short femur at his 34 week growth scan and when he was born he was perfectly normal in length. So I would try not to worry and just wait until you have more info. Good Luck
There are several women on this community who have had this sort of experience. If the sonographer told you not to worry, then don't.
If she really felt there was a serious problem, then you'd have been told that you had something serious to worry about.
I was told too that our baby had one short femur - he was fine.