Depends on pattern
This would be rare for classical trisomy 21 but could be very high for a translation 21.
I favor testing all ds infants for their chromasome pattern. If a translocation pattern is found, the parents need chromasome studies to see if one is a balanced carrier. ( ballanced 21 carriers are unaffected and have a piece of a 21 stuck on another chromasome/ but in total have all they need & no extra.