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Demystifying Down Syndrome, Part 1

A blog by Regine Lim, June 16, 2014

Down syndrome is a commonly uttered word on the OB/GYN floor. It’s not necessarily a common occurrence, but there are so many screening and diagnostic tests available, that navigating pregnancy without hearing about Down syndrome is practically impossible.

In a genetic counseling session, I like to begin with the bigger genetics topics and then zoom into detail. This bird’s-eye view is a helpful starting point to visualize the conversation in a meaningful way. In genetics, the bird’s-eye view is of chromosomes.

Chromosomes are compact, efficient storage units of our DNA, the molecule that carries our genetic information. Most human cells contain 23 pairs of chromosomes (46 chromosomes in total). Every so often, when egg and sperm combine, 47 chromosomes appear, which can be very disruptive.

Trisomy 21 is a good example: affected individuals have three copies of chromosome 21 in their cells instead of the expected two. Affected individuals typically experience mild to moderate intellectual disability; some individuals have heart defects; and some experience shortened height or low muscle tone. Trisomy 21 accounts for approximately 95% of Down syndrome cases.

A little known fact is that trisomy 21 is not genetic in the typical sense in that it’s not inherited. It’s not something that is passed down from father or mother to an affected child, but rather occurs as an error in the egg or sperm of mom or dad and shows up in all of the cells of the child.

Although we are not sure how certain factors affect the risk, we do know that as mom’s age increases, the likelihood of having a child with an extra copy of a chromosome increases. That is why so many women who will be age 35 or up at the expected time of delivery are referred to genetic counseling. It’s completely normal, and although I’m wildly biased, I think that these discussions can be tremendously valuable. If you’re invited to have one, say yes!

How can Down syndrome be detected?

Because the vast majority of Down syndrome cases aren’t inherited, they can only be detected after conception. These tests are plentiful, and they’re not all the same, which I’ll discuss in detail in the next post.

Note: In the rare cases of inherited Down syndrome, the risk may actually be determined before conception. One of the biggest clues to trigger more preconception genetic testing in a couple is if there is a family history of Down syndrome and/or a family history of multiple miscarriages. This is one example of how helpful it can be to gather a family health history.


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