A genetic disorder involving an extra chromosome 13; hence, the individual has three copies of chromosome 13. Trisomy 13 syndrome is characterized by severe abnormalities in organs and features. Infants born with trisomy 13 syndrome tend to be small at birth and to have spells of interrupted breathing, or apnea, in early infancy. Trisomy 13 syndrome is associated with severe mental retardation, deafness, microcephaly (small head), and scalp defects. Brain malformations are common, and myelomeningocele, or protrusion of the spinal cord through the vertebrae, may occur. Polydactyly, or extra fingers or toes, and cleft lip and/or palate are often present, and the ears are abnormally shaped. Undescended testicles in boys and an abnormally shaped uterus in girls are sometimes seen.