A genetic disorder involving an extra chromosome 18; hence, the individual has three copies of chromosome 18. Trisomy 18 syndrome is characterized by abnormal physical features, heart defects, and other organ malformations. Infants born with the syndrome are small, thin, and frail, and they fail to thrive. Babies with the disorder may have microcephaly, or small heads; low-set, malformed ears; cleft lip and/or palate; short big toes that bend backward; absent or small thumbs; and clenched fists with the index finger overlapping the third and fourth fingers. Congenital heart defects occur in 90 percent of cases. Other associated conditions include spina bifida; scoliosis, or curvature of the spine; deafness; kidney defects; and severe mental retardation.