Trisomy 18 is a genetic disorder in which a person has a third copy of material from chromosome 18, instead of the usual two copies.
Trisomy 18 occurs in 1 in 6,000 live births. It is three times more common in girls than boys.
The syndrome occurs when there is extra material from chromosome 18. The extra material affects normal development.
Exams and Tests
An exam during pregnancy may show an unusually large uterus and extra amniotic fluid. There may be an unusually small placenta when the baby is born.
Other signs include:
- Hole, split, or cleft in the iris of the eye (coloboma)
- Separation between the left and right side of the abdominal muscle (diastasis recti)
- Umbilical hernia or inguinal hernia
There are often signs of congenital heart disease, such as:
Tests may also show kidney problems, including:
There are no specific treatments for trisomy 18. Which treatments are used depend on the patient's individual condition.
Half of infants with this condition do not survive beyond the first week of life, and 90% of the children will die by one year of age. Some children have survived to the teenage years, but with serious medical and developmental problems.
Complications depend on the specific defects and symptoms.
When to Contact a Medical Professional
Genetic counseling can help families understand the condition, the risks of inheriting it, and how to care for the patient.
Tests can be done during pregnancy to find out if the child has this syndrome.
Genetic testing is recommended for parents who have a child with this syndrome and who want to have more children.
Summar K, Lee B. Cytogenetics. In: Kliegman RM, Stanton BF, St. Geme JW III, Shor NF, Behrman RE, eds. Nelson Textbook of Pediatrics. 19th ed. Philadelphia, PA: Saunders Elsevier; 2011:chap 76.
Reviewed By: Chad Haldeman-Englert, MD, FACMG, Wake Forest School of Medicine, Department of Pediatrics, Section on Medical Genetics, Winston-Salem, NC. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Bethanne Black, and the A.D.A.M. Editorial team.