How Do You Get Down Syndome?

Once in a while nature doesn't do things as planned and something geneticists call an "anomaly" occurs. As the mother of a child with Down syndrome, I've never been crazy about the word "anomaly." It means an occurrence that is strange, unusual, or unique. It is also defined as a deviation from an established rule. My son is unique, but I would never refer to him as strange or unusual.

For people with Down syndrome, the anomaly that happens causes the 21st set of chromosomes to get an extra one. You can see this visually in the picture above. The set circled in red is the 21st set of chromosomes. While the majority of folks in the world have only 2 chromosomes occupying their 21st set, people with Down syndrome have 3.

Because of this "extra" 21st chromosome, Down syndrome is often referred to by medical professionals as trisomy 21. Trisomy means three, so trisomy 21 means 3 of the 21 set. An interesting fact about trisomies is that trisomy 21 is not the only kind there is. Nope, there are many other trisomies. Another common trisomy, is trisomy 18, or 3 copies of the 18th set of chromosomes. Trisomy 13 is another common condition also known as Patau syndrome. These are the trisomies that exist in people because they are survivable. Trisomies of the other pairs of chromosomes occur in fetuses but many are not survivable and result in miscarriage of the fetus.

The photo of chromosomes seen above is called a "Karyotype." This is something that a geneticist prepares to diagnose Down syndrome. When I was several months pregnant with my child that has Down syndrome, I underwent an amniocentesis where they drew amniotic fluid to analyze. From this, they prepared a Karyotype that looked identical to the one in this photo.

A karyotype is a definitive test that is considered by medical professionals to be as close to 100% accurate as a test can be. There are often blood tests, such as an AFP, or markers found on an ultrasound prior to the karyotype, but these blood tests and markers are not always accurate. If there is reason for a doctor to suspect Down syndrome, either from the blood test, or from the ultrasound, they will recommend an amniocentesis for a definitive diagnosis. Amnios are controversial, because they pose a small risk of miscarriage.

Now that you know what Down syndrome is, you are probably wondering how it happens. How do you get Down syndrome exactly?

You might remember the words meiosis and mitosis from science class in school. Without going into too much detail, these words are about cell division and replication. The fertilized egg begins to replicate and duplicate itself soon after fertilization. A number of "anomalies" can occur during this process. These unique happenings cause Down syndrome. In fact, they cause three different possible types of Down syndrome.

There are 3 different types of Down syndrome? Yes, indeed. They all have different names and they all have different causes.

1) Non-Disjunction - 95% of cases - Failure of the 21st set of chromosomes in either the egg or sperm cell to divide properly.
2) Translocation - 4% of cases - Attachment of the 3rd 21st chromosome to another pair of chromosomes. Usually, but not always, hereditary.
3) Mosaicism - 1% of cases - Some, but not all cells receive an extra 21 chromosome.

If you look at the information above you will notice that only one type of Down syndrome is known to be hereditary. It is translocation and it is rare, accounting for only 4 in 100 cases. Of those cases, 3 out of 4 people who have translocation Down syndrome have inherited it; the other 1 of 4 did not.

In Mosaic Down syndrome, the trisomy occurs during cell replication and not all cells replicate with the extra chromosome. That means in people with Mosaic Down syndrome, some cells have the traditional 2 chromosomes. People with Mosaic Down syndrome are typically less significantly affected.

Back to the original question, "How Do You Get Down Syndrome?" The answer doesn't appear to be simple. It is an anomaly in cell division.

In 3% of cases it is hereditary. The medical profession doesn't know exactly what causes the other 97% of cases. Maternal age is the only known increasing risk factor, meaning that older moms having a baby do have a higher risk of having a child with Down syndrome.

However, the large majority of babies born with Down syndrome are born to young mothers. A mother's risk begins increasing slightly each year after the mom's 20th birthday. Over 80% of babies with Down syndrome are born to moms under the age of 35 years.

The statistics below come from the US National Down Syndrome Society. I'll strip away the negative connotations implied by the word "risk" and say the likelihood of giving birth to one of these precious angels at each age are as follows:

Maternal Age...Risk
20.............1 in 2000
21.............1 in 1700
22.............1 in 1500
23.............1 in 1400
24.............1 in 1300
25.............1 in 1200
26.............1 in 1100
27.............1 in 1050
28.............1 in 1000
29.............1 in 950
30.............1 in 900
31.............1 in 800
32.............1 in 720
33.............1 in 600
34.............1 in 450
35.............1 in 350
36.............1 in 300
37.............1 in 250
38.............1 in 200
39.............1 in 150
40.............1 in 100

Q: Is Down Syndrome a Disease? A: Down syndrome is not an infectious disease. However, it is an abnormal condition affecting the body.

Q: Is Down Syndrome Hereditary? A: Less that 3% of Down syndrome births are hereditary. Those that are hereditary are a result of Translocation.

Q: Are There Down Syndrome Animals? A: There are trisomies (errors in cell division) that occur in animals. Animals do not have the same pairs of chromosomes that humans do, but it is possible for an animal to have trisomies in chromosomal pairs that cause similar characteristics seen in people with Down syndrome.

Q: What are the Types of Down Syndrome? There are 3: Nondisjunction, Translocation, and Mosaicism