What Is the Karyotype of Trisomy 18?

Medically Reviewed on 5/19/2021
karyotype of trisomy 18
The karyotype in trisomy 18 is abnormal, with 3 copies of chromosome 18 instead of 2

The karyotype, or collection of chromosomes, in a baby with trisomy 18 (Edwards’ syndrome) is abnormal. An extra chromosome exists in some or all of the body's cells because instead of two copies, they have three copies of chromosome 18. This affects the growth of the baby and the development of their organs.

Trisomy 18 is rare, but the chances of having a baby with the condition increases with the mother’s age. Unfortunately, there is no way to prevent trisomy 18 in an unborn child.

What is a karyotype?

A karyotype allows doctors to look at a person’s structure or number of chromosomes. A healthy individual has 23 pairs of chromosomes, or 46 chromosomes total.

In a karyotype test, chromosomes are extracted from the blood, most commonly from white blood cells, and examined under a special microscope. A trained cytogeneticist, who is a specialized doctor in genetics, can look for missing or extra pieces of a chromosome to determine whether abnormalities like trisomy 18 exist.

Each chromosome is numbered based on its size, with chromosome 1 being the largest, and chromosome number 23 being the smallest. Chromosome number 18 is one of the smallest chromosomes.

What are the types of trisomy 18?

Symptoms of a baby with trisomy 18 depend on which type they have:

  1. Full trisomy 18
  2. Mosaic trisomy 18
  3. Partial trisomy 18

Full trisomy 18

  • Babies with full trisomy 18 have an extra copy of chromosome 18 in all cells.
  • Symptoms of this type are the worst among all the three types. 
  • Most babies with full trisomy 18 die in their mother’s uterus before they are even born.
  • Accounts for about 95% of cases.

Mosaic trisomy 18

  • Babies with mosaic trisomy 18 have an extra copy of chromosome 18 in just some cells. 
  • Most babies with this type of trisomy 18 survive up to one year of life. Some can even reach adulthood.
  • Accounts for about 5% of cases.

Partial trisomy 18

  • Babies with partial trisomy 18 only have a part of an extra copy of chromosome 18 and not its whole copy in the cells.
  • Accounts for less than 1% of cases.

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Medically Reviewed on 5/19/2021
What Is Trisomy 18? https://www.webmd.com/baby/what-is-trisomy-18#1

Edwards syndrome (trisomy 18). https://www.nhs.uk/conditions/edwards-syndrome/