My sons arms and legs always measured behind during my pregnancy and he's fine, perfectly normal
I had a scan at 40+5 and his arms were measuring at 37 weeks and his femurs at 36.5 weeks. He always had a kind of large head too, it was in the 75th percentile at birth, while his length was the 50th and his weight the 35th.
He's just got my bodytype, I'm short with very short limbs. He's a normal baby and in the 25th percentile for length, weight and head cirumference now, but still has short legs.