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.

What is the likely outcome for a baby diagnosed with these conditions?

Parents who receive a diagnosis for their baby are faced with making complex and difficult decisions.  You may be asking yourself what the quality of life will be like for your baby.  You may also be considering what the impact will be on the whole family as you care for your affected child.

Parents often feel that there is little time to gather the information they need to make such a life-changing decision. Download a copy of the Your Unborn Baby handbook.

Sadly, the risk of miscarriage and stillbirth is high but of the babies who do survive pregnancy, almost 1 in 10 live to celebrate their 1st birthday.

Some of these children thrive for many years, even living into adulthood. Read our Family Stories for information.

Infants with less common mosaic and partial forms tend to survive for longer - your local genetic service can advise you on an individual karyotype.

In Britain, more than 6 in every diagnosed pregnancies will be terminated early.  Read more about termination here.

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