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What is Trisomy 18?

By D. Jeffress
Updated Mar 03, 2024
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Trisomy 18 is a congenital disorder that results from an extra copy of chromosome 18. The third chromosome severely impairs organ and brain development in the fetus, and often leads to fatal complications within the first few weeks of life. Infants who survive into childhood are often physically deformed and have serious cognitive deficiencies. The primary focus of treatment for a patient with trisomy 18 is to reduce pain and repair structural anomalies of the heart and lungs, giving the baby the best possible chances of surviving past infancy.

Healthy babies are born with 23 pairs of chromosomes, receiving equal numbers from each parent. An infant who suffers from this disorder acquires an additional copy of the chromosome from one parent, usually the mother, due to a second round of meiosis during the earliest stages of development. Trisomy 18 is the second most common chromosomal defect, though it is much rarer than trisomy 21, the disorder responsible for Down syndrome. It is about three times as likely to affect female fetuses than males.

Symptoms that are commonly seen in an infant with trisomy 18 include a smaller than average head, low birth weight, and a concave chest cavity. Babies often have congenital defects that impair their ability to feed and breathe without medical assistance. Internal organs may be displaced or deformed, and most infants have underdeveloped brains. It is also common for a baby to have constantly clenched hands, crossed legs, and oddly shaped feet. Additional physical symptoms vary from patient to patient.

Obstetricians may be able to notice signs of trisomy 18 before a baby is born via ultrasound images. After an infant is born, genetic testing of a sample of skin or blood cells can confirm the presence of an extra chromosome. Once the disorder has been diagnosed, specialists can create a custom treatment plan based on the infant's specific problems. The baby may need to receive emergency surgical procedures to repair congenital heart, lung, kidney, or gastrointestinal tract disorders. He or she is often fitted with breathing and feeding tubes and kept in the hospital for several months for careful monitoring.

A baby who survives the first few months of life may be able to go home with the parents. He or she will need to receive regular checkups throughout childhood and take daily medications to avoid potentially fatal complications. Very few people with the disorder survive past adolescence, and those who do reach adulthood are severely mentally and physically handicapped, requiring constant attention and assistance with daily activities.

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Discussion Comments
By anon269946 — On May 20, 2012

I have a son with full Trisomy 18. He is 29 months old, breathing on his own and is an absolute joy in my life! It has been a long hard road, but my son is worth every tear and every smile! I could not be happier with him!

By ElizaBennett — On Dec 16, 2011

@jennythelib - I'm so sorry for what your friend went through. I hope they have found peace and that they will one day add more healthy babies to their family, if that is what they want.

Some people think it's better not to have prenatal testing if you know that you would not abort anyway. It sounds like your friend and her husband, though, were able to better prepare themselves for what was ahead. Instead of the bait-and-switch of expecting a healthy baby who simply isn't coming, they knew what to expect. Do you think that knowing was helpful for them?

By jennythelib — On Dec 16, 2011

Although there are some miraculous exceptions, the medical community considers trisomy 18 syndrome to be "incompatible with life" (as opposed to less serious chromosomal abnormalities, like Turner syndrome).

My old college roommate gave birth to a child with this terrible condition. Because she was fairly young and had no risk factors, she didn't find out anything was wrong until her 20 week anatomy scan. You know, the one where usually you find out the sex of your baby; it's a happy occasion for most people.

As she was so far along in her pregnancy, she decided to carry to term, but essentially she was preparing for both birth and death. Her baby was a boy, which meant that he was particularly unlikely to survive. (All of the people with trisomy 18 who have survived into adulthood have been females.)

She went into labor a bit early, around 36 weeks, and she, her husband, and their toddler daughter spent twelve hours with their little boy before he died.

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