Trisomy 18, or Edwards syndrome, occurs in about 1 in 5000 births. Characteristics include multiple birth defects and severe mental retardation. Most pregnancies with this syndrome end in miscarriage, and those babies that are born usually die shortly after birth. Trisomy 18 is almost always sporadic, due to a extra chromosome 18 by accident in either the egg or sperm.
Smith-Magenis syndrome (SMS) is a rare microdeletion syndrome characterized by mutiple birth defects, specific facial features, and mental retardation. Individuals with Smith-Magenis can have severe behavior problems such as self-mutilation or other complusions. This syndrome is due to a deletion of a small part of chromosome 17.