My daughter just had a miscarriage in her 14 week of pregnancy because the baby had Trisomy 18 Syndrome. However, the growth/life probably ceased at 12 weeks. What are the chances/statistics of having another child/pregnancy affected as well?

-A curious adult from Oregon

June 6, 2012

I am sorry to hear about your daughter’s miscarriage. Without having more information about the family history or the mother’s age, I can’t give an estimate of the chances that Trisomy 18 might happen again.

What I can say is that most trisomies like this are totally random.  And that a woman’s chances for a trisomy pregnancy mostly depend on her age.

So a 25 year old mother has around a 1 in 476 chance for a trisomy pregnancy while a 45 year old mother has a 1 in 20 chance.  In most cases, having had a previous trisomy pregnancy doesn’t affect these numbers very much at all.  There might be a slightly higher risk but it won’t be very much.

This isn’t always the case though.  There are a few specific cases where the chances can be much higher.  Fortunately, these are pretty uncommon.

I think you can begin to understand why it is so important to talk with a genetic counselor about these sorts of things.  He or she can help you figure out your actual odds.

What I want to do for the rest of the answer is focus on what trisomies are and how they happen.  Then I’ll end with some of the ways that you can have an increased risk for having a trisomy pregnancy.   

Trisomy Means an Extra Chromosome

Chromosomes are structures in our cells that hold our DNA or genetic information. Normally a person has 23 pairs or 46 chromosomes in their cells; they get half from mom and half from dad.  So each parent usually gives their baby 23 chromosomes.

Chromosomes are numbered 1 through 22. The 23rd pair is our sex chromosomes, X and Y.  Chromosomes are numbered based on their size with chromosome 1 being the largest and chromosome 21 the smallest.

Trisomy 18 means that there are 3 copies of chromosome number 18. Most babies that have Trisomy 18 will miscarry. Others will not live very long after being born.

Chromosomes Divide

Eggs and sperm have 23 chromosomes so that when they combine, they end up with the usual 46.  To get there, cells need to go through a complicated process called meiosis, which is outlined below for a single pair of chromosomes:

In the top cell, there are four copies of a chromosome.  As you can see, the cell splits into two cells that have a pair of chromosomes and then each splits again into two cells. 

Now the four cells at the end each have a single chromosome.  This happens for all 23 pairs of chromosomes.

Sometimes something goes wrong and a person can have an extra or missing chromosome. I’ll use this picture here to describe one way this can happen.

If you look at this picture you can see that the blue chromosome did not separate the way it should. You can see that this means that one cell now has an extra copy of the blue chromosome, while the other cell has no copies.

So when something like this happens, and a mother or father passes an extra chromosome to their baby, the baby will have 3 copies of that chromosome.  If the chromosome is number 18, then a baby can have an extra chromosome 18. This is what happens in Trisomy 18.

This can also happen with other chromosomes. For example, if this happens with chromosome 21, then a baby will have Down syndrome.

This is how it happens in most cases.  And when this mistake in separation happens, it happens by chance. There is nothing that the mother or father could do to cause it or prevent it.

Figuring Out Your Risk

There are many factors that can affect a woman’s chances of having a second trisomy pregnancy.  As I said in the introduction, the key factor is mom’s age.  The older she is, the more likely it is that non-disjunction will happen. You can see how the risk increases in the table below. (Click here for some reasons why this might be.)

For most women, this will be the information that matters.  But a few women will be at a higher risk because they are having trisomy pregnancies for a different reason than non-disjunction.  Luckily these are not very common.

I won’t have time to go into a lot of detail on all these special cases but what I can do is list a few and how common they are.  I will provide additional links if you want to explore each in more detail.

Gonadal or Germline Mosaicism: There is a small chance that some people may have eggs or sperm that already carry the wrong number of chromosomes.  This can happen in some people who don’t have the wrong number of chromosomes in other cells of their bodies.  

This can increase the chances of having another child with a trisomy since a rare, random, non-disjunction event doesn’t have to happen.  Their eggs or sperm are preloaded with the extra chromosome. 

If you spend some time on the web, you’ll see numbers that suggest that a woman is at a higher risk for a second trisomy pregnancy.  For example, a 40 year old woman’s chances may go up from 1.5% to 2.8%.

This increase is taking into account this mosaicism.  Some current research takes this into consideration and shows that you can multiply the mother’s chance based on her age by 1.8 to get the new risk number.

This is where the increased risk for the 40 year old woman came from (1.5% X 1.8 = 2.8%).  But it is important to remember that this number is for the general population…it may not apply to you.

Women with germline mosaicism have a higher than 2.8% risk.  And women without this condition have the usual 1.5%.  So the 2.8% number may not reflect your exact situation.

Balanced Translocation: Around 0.1% of the population has a balanced has no effect on the person but increases risks for trisomy.  They basically have two of their chromosomes stuck together.  This causes the two to travel together when a sperm or egg is made.

Their risk for having a pregnancy that involves trisomy can be as high as 15% but this depends on which parent carries the balanced translocation.  This is another way for a woman to be at a higher risk for a second trisomy pregnancy but this is different than gonadal mosaicsism because we have tests that can look for this.

So, as you can see there is not a simple explanation for this. This is why it is so important to see a genetic counselor to figure out your risk.  A genetic counselor can give you a more personalized risk based on your genetics and family history.

It is also important to remember that there is always a 3% risk of any type of birth defect or disorder occurring in a pregnancy for any person at this time, we don’t have a test that can look for everything.

Great animation on how non-disjunctions happen.

By Nadine Rayes, Stanford University

The risks for a second trisomy pregnancy mostly have to do with the age of the mother.

Trisomy means an extra chromosome. 

This person will be at a higher risk for a baby with Down syndrome because chromosomes 14 and 21 are stuck together.