Extra or Missing Chromosomes

What could possibly cause a chromosome disorder like trisomy 18? I understand how it happens, but are there environmental factors that can cause the presence of an extra chromosome?

-A curious adult from Illinois

July 23, 2008

Scientists have yet to pinpoint any specific thing that increases the risks for trisomy 18 except for mom's age. There are hints that other things might increase the risks (see below). But these studies are very early and aren't always supported by other studies.

What scientists do know is that chromosomal problems like trisomy 18 are a natural byproduct of how complicated making a new person is. These sorts of things happen through no fault of the mom or dad.

Trisomies are Common

As you've correctly noted, a trisomy means having an extra chromosome. People normally have two copies of each of their 23 chromosomes -- one from mom and one from dad. This gives most people a total of 46.

Sometimes something goes wrong when an egg or sperm is made. Or when cells divide in a growing embryo. Sometimes this something can be that an extra chromosome goes along for the ride. These folks now have 47 chromosomes instead of the usual 46.

Having an extra chromosome is surprisingly common. Around 0.3% of live births, or 3 in 1000, has an extra chromosome. And this is really just the tip of the iceberg.

Scientists estimate that at least 4% of all pregnancies have an extra chromosome. Most of these don't survive to birth because having an extra chromosome is usually fatal.

But some trisomies survive to birth. Trisomy 18 (Edwards syndrome) and trisomy 21 (Down syndrome) are two examples where babies can be born with an extra chromosome.

Trisomy Risk Factors

So what are the risk factors for having a child with trisomy 18? Just like all the other trisomies, the most significant risk factor is maternal age. Click here to learn more about the reasons why older women are more likely to have a child with Down syndrome.

There are no other known factors that can definitely affect trisomy 18. But there are hints that a few things might increase the risk for some people. These risks don't look significant so far but they might teach us something about how and/or why trisomies happen.

Let's look more closely at a couple of these studies. A few have shown that there is a seasonal effect with different trisomies. What this means is a higher percentage of children are born with trisomy during certain months.

In Kuwait, almost half of the babies with trisomy 18 are born in the spring months (March through May). This trend was also seen in Canada, but not England or Denmark.

So what does this mean? Well, we're not quite sure. Some scientists think this means there may be some environmental effects in these regions that can explain these differences. It could also mean that certain seasons affect female hormones and egg development.

Seasons aren't the only things that may affect trisomy. One study found that trisomy 18 and 13 were higher in urban environments. This also suggests environment may have an effect. But again we're not exactly sure how.

As you can see, scientists are still unclear if environment has any real effects on trisomy. The only true risk factor found for trisomies is the mother's age.

Jackie Benjamin, Stanford University

Trisomies, like trisomy 21, are more common
than people might think.