Conditions

Trisomy 18 is a rare chromosome abnormality, affecting about one in every 3,000 to 4,000 babies. This condition involves severe mental retardation as well as physical birth defects. Most babies with trisomy 18 survive a short period of time after birth, just a few days or weeks. About one out of ten babies born with trisomy 18 will live to the time of their first birthday. A very small minority of children with trisomy 18 can live for many years.

Babies with trisomy 18 are usually small for their age, have a poor suck, and a weak cry. It is difficult for babies with trisomy 18 to coordinate breathing, sucking, and swallowing, so feeding problems are common. 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 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.

Cause
Trisomy 18 is caused by an extra chromosome, specifically an extra copy of chromosome number 18. Chromosomes are the packages of genetic material which provide the instruction for growth and development. Normally, the chromosomes are paired, one in each pair from the mother, and one from the father.  The presence of three copies of chromosome 18, instead of two, interferes with the instructions for growth and development in the baby. 

The extra chromosome 18 is present at the time of conception. It occurs as a random event when either the egg or sperm is being formed. There is nothing known to either cause or prevent this from happening. As a woman’s age increases, she is more likely to have a baby with a chromosome abnormality, but these types of birth defects can happen at any age.

Occasionally trisomy 18 is present in only some of the baby’s cells; this is known as mosaic trisomy 18. In these cases the medical problems may be milder than full trisomy 18, but there is a wide range of variability.

Diagnosis
A chromosome study, either by prenatal testing or by blood test after birth, can accurately diagnose trisomy 18. Testing can also show whether the baby has full or mosaic trisomy 18.

Prenatal screening tests and prenatal ultrasound can sometimes raise concerns about the possibility of trisomy 18. In order to confirm a diagnosis of trisomy 18, cells from the fetus are needed for evaluation. Trisomy 18 can be diagnosed prenatally by having CVS or amniocentesis.

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.

Treatment/Services
When a baby is born with trisomy 18, parents usually need to make some difficult decisions about whether to use medical interventions to prolong the baby's life, or whether to simply give the baby supportive care to make him or her as comfortable as possible. You can receive help and support in this process from the baby's doctor, the nursery social worker, hospital chaplain and your genetic counselor.

RESOURCES

Genetic Home Reference: Trisomy 18 - General information about trisomy 18 provided by the National Library of Health. Also includes links to many related sites.

Trisomy 18 Foundation - Site provides information about trisomy 18 and support resources for parents and families.

SOFT (Support Organization for trisomy 18, 13, and Related Disorders) - Support group for parents who have had or are expecting a child with a chromosome disorder (especially Trisomy 18, and Trisomy 13).