trisomy 21

Trisomy 21 is one of the most frequent chromosomal abnormalities that can be detected with an amniocentesis.

Chromosomal abnormalities are malformations in the baby’s chromosomes, which are the structures that carry genetic information to all the baby’s cells. There are many different chromosomal abnormalities – keep in mind that we have 23 pairs of chromosomes and that there are different types of abnormalities, so imagine how many conditions can result from them. Most of them end up in early miscarriages and can’t even be detected by doctors.

However, chromosomal abnormalities are not common. Trisomy 21 is one of the most frequent (but it is still rare), and it results in Down Syndrome. Don’t miss the following lines to learn everything about it.


What is trisomy 21?

It is one of the most common chromosomal abnormalities in babies, affecting around 1,3 out of 1.000 births. In the United States, around 5.000 babies with Down Syndrome are born every year.

Scientifically, trisomy 21 means that one of pairs number 21 of chromosomes will have three instead of two (thus the name ‘trisomy’), which means that the baby’s cells will have a total of 47 chromosomes instead of the 46 they’re supposed to have. This abnormal genetic information will be reproduced in all the cells of the baby, which will translate into some differences in their body and their brain – which will see in detail below.


How is trisomy 21 detected?

Some of the regular screening tests carried out during pregnancy detect the possibility of the baby having trisomy 21. That possibility can be detected by the anomaly scan, paired with a blood test. An abnormal result in this test doesn’t mean that your baby is affected by trisomy 21, but that there is a possibility that he is. The doctor will have to order a diagnosis test to find out if the condition is present, which can be:

  • The amniocentesis test: It analyses a sample of the amniotic fluid which is collected by inserting a needle through your tummy.

  • The Chorionic villus sampling: It analyses a small sample of cells from the placenta or the chorionic villi by inserting a needle through your belly.

  • PUBS: The PUBS or percutaneous umbilical blood sampling, also called cordocentesis, is a Down Syndrome test consisting in removing a sample of foetal blood by inserting a needle into the umbilical cord.

In some cases, the doctors will want to perform one of those tests, usually the amniocentesis, whatever the results of the anomaly scan.


Are there risks factors?

Yes, there are some risks factors for your baby having trisomy 21 – which will mean that you have the right to have an amniocentesis test whatever the results of your routine anomaly scan.

The three main risk factors are the following:

  • The age of the mother: The risk of trisomy 21 is higher as the woman’s eggs age, which is why the amniocentesis will be offered to all pregnant women over 35.

  • A carrier parent: People with Down Syndrome don’t reproduce often due to the fertility issues that come with the condition. But among the small percentage of women with Down Syndrome who do, 50% of them will have a child with trisomy 21 as well. People with translocation trisomy 21 should be careful, because they have a higher risk.

  • Having another child with Down Syndrome.


What does it imply for the baby?

Trisomy 21 means that your baby will be born with Down Syndrome, which means that he or she will have several recognisable physical traits, such as a flat face and a small head, small hands and feet, poor muscle tone and almond-shaped eyes, among others. Their mental development will be slower and they will have a lower IQ, but they can still have optimum social and creative skills. 

Trisomy 21 also translates into some health conditions that come with Down Syndrome, such as heart disease, celiac disease, intestinal and thyroid problems, eye and hearing problems and early dementia. Besides, their immune system also suffers from several abnormalities, which will make them more susceptible to infectious diseases.

However, children with Down Syndrome are not all the same. They are entirely different from one another, and they will inherit almost all traits from their parents.


Are there different types of trisomy 21? What are the differences?

Yes, there are three different forms of trisomy 21 variations. The most common, just called Trisomy 21, is the one we’ve explained: There is an extra chromosome 21. It is the one present in 90% of babies with Down Syndrome. The other two are rare:

  • Translocation Trisomy 21: Present in around 4% of the cases, it happens when part of a chromosome 21 is attached to another chromosome, so the person affected will have 45 chromosomes instead of 46. It’s the only case of trisomy 21 that doesn’t result in Down syndrome, since the baby affected will still have the right amount of genetic material. However, the risk for the parents to conceiving a baby with regular trisomy 21 in the future will be be higher.

  • Mosaic Trisomy 21: The least common of all three (less than 2% of cases). The difference is that there is an abnormality in the reproduction of the cells, so that extra chromosome 21 is present in some of the cells, but not all of them.


What should I do if the results of trisomy 21 testing are positive? 

That is entirely up to you and your partner. In the United Kingdom, abortion is legal, and that is an option you have the right to take in the case of trisomy 21. Keeping your baby or not is a personal decision that you will need to make take considering your own views, principles and priorities, and whatever you decide should concern you and your family only.