Chromosome 18p duplication is a rare chromosome abnormality in which there are three copies of the short (p) arm of chromosome 18 in each cell, rather than the usual two copies. The severity of the condition and the signs and symptoms depend on the size and location of the duplicated genetic material and which genes are involved. A complete duplication of the p arm of chromosome 18 is rare. Reported features include: intellectual disability, foot or hand anomalies, distinctive facial features, and epilepsy. Chromosome 18p duplication may occur sporadically in a family with no previous history or may be inherited from a parent with the same duplication or another chromosome abnormality, such as a balanced translocation. Treatment is based on the signs and symptoms present in each person.

Some researchers have suggested that several of the cases reported in the medical literature may represent cases in which other chromosome changes are present but are not yet identified. This may help explain the variability in the signs and symptoms present. It is additionally suspected that some individuals with chromosome 18p duplication may not be diagnosed if they have mild or absent signs and symptoms and are of normal intelligence.

It is essential to work with one's person's health care provider and medical team to determine the appropriate medical management plan.