Trisomy 18 is a syndrome associated with the presence of a third (extra) number 18 chromosome.
Causes, incidence, and risk factors
Trisomy 18 is a relatively common syndrome affecting approximately 1 out of 3,000 live births and affecting girls more than three times as often as boys. Multiple abnormalities are associated with the presence of an extra number 18 chromosome. Many of these abnormalities are not compatible with more than a few months of life. Few infants survive beyond the first year.
Common findings include low birth weight, mental retardation, low-set ears, malformed ears, small jaw (micrognathia), hand abnormalities, congenital heart disease, hernias, and undescended testicle (cryptorchidism). There may be many other abnormalities noted.
- Unusually large uterus during pregnancy
- Low birth weight infant
- Mental deficiency
- Low-set ears
- Small jaw (micrognathia)
- Clenched hands
- Hypoplastic (underdeveloped) fingernails
- Umbilical hernia or inguinal hernia
- Diastasis recti
- Crossed legs (preferred position)
- Congenital heart disease o VSD (Ventricular septal defect) o ASD (atrial septal defect) o PDA (patent ductus arteriosus)
- Congenital kidney abnormalities o Horseshoe kidney o Hydronephrosis o Polycystic kidney
- Coloboma of iris
- Motormental retardation
- Pectus carinatum
Signs and tests
Examination of the pregnant woman may show polyhydramnios (extra amniotic fluid). At the birth of the child, an unusually small placenta may be noted.
Physical examination may show an excess of arched type finger print patterns. X-rays may reveal a short breast bone (sternum). Chromosome studies show trisomy 18, partial trisomy, or translocation.
Treatment is supportive, but life-sustaining measures are not recommended.
Support Organization for Trisomy 18, 13 and Related Disorders (SOFT)
2982 S Union St
Rochester, NY 14624
Trisomy 18 Support Foundation, Inc.
4301 Connecticut Ave. N.W. Suite 404
Washington, D.C. 20008-2369
The abnormalities of trisomy 18 are generally not compatible with more than a few months of life. Fifty percent of the affected infants do not survive beyond the first week of life. More than 10 children have survived to teenage years, but usually with marked handicaps.
Complications depend on the specific abnormalities that affect the infant.
Calling your health care provider
Call your health care provider and genetic counselor if you have had a child with Trisomy 18 and you plan to have another child.
Prenatal diagnosis of trisomy 18 is possible with an amniocentesis and chromosome studies on amniotic cells. Parents who have a child with translocational trisomy 18 should have chromosome studies, because they are at increased risk for another child with trisomy 18.
by Armen E. Martirosyan, M.D.
All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.