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What Is Trisomy 18?

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You might have heard about trisomy 18 in the news, or your doctor may have told you that your unborn baby has this condition.

Here are answers to common questions about trisomy 18, including what causes it, how it's diagnosed, and how it affects babies.

Trisomy 18 Explained

Trisomy 18 is a chromosomal abnormality. It's also called Edwards syndrome, after the doctor who first described it.

Chromosomes are the threadlike structures in cells that hold genes. Genes carry the instructions needed to make every part of a baby's body.

When an egg and sperm join and form an embryo, their chromosomes combine. Each baby gets 23 chromosomes from the mother's egg and 23 chromosomes from the father's sperm -- 46 in total.

Sometimes the mother's egg or the father's sperm contains the wrong number of chromosomes. As the egg and sperm combine, this mistake is passed on to the baby.

A "trisomy" means that the baby has an extra chromosome in some or all of the body's cells. In the case of trisomy 18, the baby has three copies of chromosome 18. This causes many of the baby's organs to develop in an abnormal way.

There are three types of trisomy 18:

  • Full trisomy 18. The extra chromosome is in every cell in the baby's body. This is by far the most common type of trisomy 18.
  • Partial trisomy 18.The child has only part of an extra chromosome 18. That extra part may be attached to another chromosome in the egg or sperm (called a translocation). This type of trisomy 18 is very rare.
  • Mosaic trisomy 18. The extra chromosome 18 is only in some of the baby's cells. This form of trisomy 18 is also rare.

How Many Babies Have Trisomy 18?

Trisomy 18 is the second most common type of trisomy syndrome, after trisomy 21 (Down syndrome). About 1 in every 5,000 babies is born with trisomy 18, and most are female.

The condition is even more common than that, but many babies with trisomy 18 don't survive past the second or third trimester of pregnancy.

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