Posted October 30, 2008on:
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. 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.
Common findings include
- 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
- Congenital kidney abnormalities
- Coloboma of iris
- Motormental retardation
- Pectus carinatum
Signs and tests
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.
Calling your health care provider
Prenatal diagnosis of trisomy 18 ispossible 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.
Update Date: 8/19/2003
Updated by: Douglas R. Stewart, M.D., Division of Medical Genetics, Hospital of the University of Pennsylvania, Philadelphia, PA. Review provided by VeriMed Healthcare Network.
Trisomy 18 Disorders Menu
Trisomy 18 Support
Trisomy 18 is a rare chromosome abnormality, affecting about one in every 3,000 to 4,000babies, which involves severe mental retardation as well as physical birth defects. Most babies with this condition do not live longer than a few days, weeks, or months; about 70% of fetuses conceived with Trisomy 18 die before birth. Those who do survive are usually small for their age and have a poor suck and weak cry. They have feeding problems, since it is difficult for them to coordinate breathing, sucking, and swallowing. Only about 10% are alive at the time of their first birthday, although some children do live for years. Children with Trisomy 18 who survive past infancy usually do not walk, but some have learned a few words or signs to use in communicating. Some children with Trisomy 18 smile responsively and interact with family members.
In addition to mental retardation, babies with Trisomy 18 have various birth defects. The most common of these are heart defects, a cleft lip (when the upper lip does not form properly), and kidney problems. The babies are often born with their fists clenched; this is due to abnormalities in the nervous system which prevent the hand muscles from receiving proper instructions to move normally.
The problems with Trisomy 18 are caused by anextra chromosome — specifically a third copy of chromosome number 18. Chromosomes, which normally come in pairs – one in each pair from the mother and one from the father — are the packages of genetic material which give the baby the instructions for growth and development. The extra genetic material that a baby with Trisomy 18 receives interferes with these instructions and causes abnormalities in many organs of the body. The extra copy of chromosome 18 was present in either the sperm or the egg which went to form the baby, and this mistake occurred either prior to or just at conception. Many parents wonder why Trisomy 18 occurred, but we know of nothing that either parent could have done that would have caused or prevented this from happening.
It is very unusual for a family to have a second child with Trisomy 18. If you were under 30 years old when pregnant with your baby with Trisomy 18, then there is approximately a 1% chance for another baby to have Trisomy 18. If you were over 30, then your risk is not increased; it is simply the same as that of any other woman your age. Prenatal diagnosis is an option for you in a future pregnancy, even though the risk of recurrence is low.
Mosaic Trisomy 18
Occasionally Trisomy 18 is present in only some of the baby’s cells; this is known as mosaic Trisomy 18; in these cases the outlook is often better than for those babies with full Trisomy 18.
Trisomy 18 can be diagnosed prenatally through CVS or amniocentesis (See CVS/Amnio section) ; it is also one of the conditions screened for in the XAFP program (See XAFP section). Sometimes ultrasound findings provide the first indication that the baby has a problem.
In other cases a baby with Trisomy 18 may be diagnosed shortly after birth. If facial features, a heart defect, or other problems raise the possibility of Trisomy 18, a blood chromosome study will be done to confirm the diagnosis. If a baby is born with Trisomy 18, decisions about care and interventions (such as heart surgery) will have to be made and should be discussed with your baby’s doctor.
If you have just learned during your pregnancy that your baby has Trisomy 18 and are trying to decide whether or not to continue your pregnancy, you will probably not find many sources of information on this condition. There is an organization called SOFT (Support Organization for Trisomy 18, 13, and Related Disorders) which is a group composed of parents who have children with Trisomy 18 and other related conditions, as well as professionals who have an interest in caring for children with those conditions. These parents are strong advocates for their children and emphasize the positive aspects involved in raising a child with Trisomy 18. Parents who continue a pregnancy with a baby with Trisomy 18 or who have a baby born unexpectedly with this condition have found this group to be a great support. Their website can be found at