Trisomy 21
NIPTTrisomy 21Down's Syndrome

Trisomy 21

January 20, 2021


What is Trisomy 21?

Trisomy 21 is also called Down’s Syndrome. It is one of the most common chromosomal abnormalities in humans, and is caused by the presence of an extra chromosome 21 in the cells of the baby.

The degree that the extra chromosome 21 affects the baby is highly individual. Every person with Down’s Syndrome is unique and has their own personality. Most people with Down’s Syndrome will have some form of learning disability. While some will grow up to be more independent, others will need more regular care. Some people with Down’s Syndrome will also have complications in the form of hearing problems and heart conditions. Today, most people with Down’s Syndrome live into their fifties and sixties with a small number of people living into their seventies.

Down’s Syndrome does not run in families. The chances of having a baby with Down’s Syndrome is somewhat higher for older mothers.

Why is this analysis important?

A screening test for Down’s Syndrome can help you make choices about further tests and care or treatment during your pregnancy or after your baby's born. Currently, all pregnant women are able to have a combined serum screening test which consists of a blood test and an ultrasound scan via the NHS, but this test is less sensitive than the non-invasive prenatal test (NIPT), which means that the ability of the NIPT test to correctly identify the condition is higher. The combined serum screening test has a sensitivity of 86% for Down’s Syndrome, while the NIPT test is almost 100% sensitive for this condition.


The results can either show a “low chance” or an “increased chance” of Down’s Syndrome.

If the result shows “low chance”, this means that it is very unlikely that your baby is affected by Down’s Syndrome.

If the result shows “increased chance”, then you should discuss with your midwife the need of an invasive diagnostic test, such as chorionic villus sampling (CVS) or amniocentesis, to confirm the results. CVS is usually carried out between the 11th and 14th weeks of pregnancy, while amniocentesis is carried out between the 15th and 20th weeks of pregnancy.

Other Considerations

The NIPT test is a screening test and can only tell you if your baby has an increased chance of Down’s syndrome or not, but cannot make a definite diagnosis. It only screens for Trisomy 21, 18 and 13, but not other genetic conditions that can be present at birth. The screening chances have been calculated based on information you have provided via our questionnaires. Any inaccuracies in the information provided could alter the chances of these genetic conditions being present.


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