I have just had my 20 week scan and the nuchal fold was 6.3mm, does this mean my baby has downs?

No: The nuchal fold was developed to be assessed at 11-13 weeks, not 20. Even when it is increased, it doesn't mean the baby the baby has down's, just that there is increased risk it could.

Not necessarily: 6.3 mm is above the 6 mm cut-off but that doesn't mean the fetus has trisomy 21. At your age however, an amniocentesis would probably be suggested to determine if there is a chromosomal abnormality due to the increased risk seen in older mothers. Did the ultrasound suggest any other abnormal markers like short long bones or facial abnormalities? Good luck.