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Detailed Definition Trisomy 18 is a rare chromosome abnormality, affecting about one in every 3,000 to 4,000 babies, 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 |
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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 an extra 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 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. 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; it is also one of the conditions screened for in the XAFP program. 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.
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.
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 http://www.trisomy.org. |
