What Is Trisomy?



Trisomy 13Trisomy 18

Trisomy is a chromosomal disorder characterised by an additional chromosome, so your baby has 47 instead of 46. Down's Syndrome (Trisomy 21), Edwards' Syndrome (Trisomy 18) and Patau's Syndrome (Trisomy 13) are the most common forms of Trisomy.

This extra genetic material affects foetal development.  This syndrome is not a disease and, except in rare cases, it is not a hereditary condition.  Instead, Trisomy occurs by chance at conception.

Although it's been more than 50 years since researchers first identified the chromosomal nature of Trisomy 13 and 18, we still do not know what causes the presence of an extra chromosome.  It is not something you have done or not done - it can happen to anyone.

What is the risk of a pregnancy being affected?

  • About 1 in 200 births result in a baby with a chromosome defect.
  • The age of the mother is a major known risk factor for having a baby with Trisomy 13 or 18.
  • There is a small increased risk of the same Trisomy after a previous pregnancy with Trisomy 13 or 18.  There may also be a small increased risk for women with a previous pregnancy with Trisomy 21 (Down's Syndrome).
  • Except in rare inherited cases, chromosome defects are usually one-off events.

You may wish to discuss the option of prenatal diagnosis if you become pregnant in the future or if you have had a previous pregnancy with a Trisomy.

How do these conditions affect children?

Trisomy 13, 18 and related conditions cause a variety of developmental and health difficulties for affected children. Parents who receive a diagnosis for their baby are therefore  faced with making complex and difficult decisions.  

Trisomy 13 and 18 can affect how long a baby will live for.  A summary of the most recent research on this can be found here.

SOFT UK produces a number of booklets to provide information to families about the conditions:

Your Unborn Baby Handbook.

Your Baby Handbook



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