Chromosomal. Edwards syndrome is usually due to trisomy 18 (extra copy of chromosome #18) that is an accidental occurrence with low recurrence risk (<1%) to the parents. Occasionally a translocation is found and the parents need testing to see if they have a balanced translocation (no symptoms) with higher recurrence risk.
Varies. Edward's syndrome, trisomy 18, occurs when the baby has three copies of chromosome18. If three full copies so 47 chromosomes, recurrence is less than 1percent. Some babies, however, have a normal number of chromosomes but an extra 18 because of a translocation (a piece of 18 traded with another chromosome and led to trisomy) The recurrence risk is higher and both parents will be tested .