A severe chromosomal disorder caused by the presence of an extra #18 chromosome. Physical findings may include: characteristic face and hand positioning, short sternum, small pelvis with limited hip mobility, rocker-bottom feet, congenital heart defects, hypertonia, and growth and mental retardation. If liveborn, death usually occurs by 6 months, although some children may survive for years.

or Edward's syndrome caused by meiotic nondisjunction; major clinical findings include mental retardation and periarticular limitation of motion of distal interphalangeal joints.

Edward's syndrome or trisomy 18 is a chromosomal abnormality, which is more severe and less common than Down's syndrome. Symptoms include severe learning disabilities and often numerous defects, such as cleft lip and palate, club foot, and malformation of internal organs.