Today we’re going to talk about trisomy 18.
I did a post a while back with Kristen, whose baby Hannah had trisomy 18. And in the comments, I noticed a lot of conversation around how we diagnose this condition and what survival is like, and what the quality of life is like for babies who have trisomy 18.
Since there’s such interest in this topic, let’s talk more about how we diagnose trisomy 18 and what life expectancy looks like for babies who have trisomy 18.
What is trisomy 18 or Edwards Syndrome?
Trisomy 18 or Edwards syndrome happens when babies have an extra 18th chromosome. Most of us are born with 46 chromosomes, 23 from mom and 23 from dad. Some babies, though are born with an extra chromosome.
Babies with Down syndrome have an extra 21st chromosome in every cell of their bodies. Babies who have trisomy 18 are born with an extra 18th chromosome. When I was training 20 years ago, trisomy 18 was considered a uniformly lethal condition. So when we gave the diagnosis of trisomy 18 to parents, more than half of them elected not to continue the pregnancy.
What we know now about trisomy 18 is that it’s a life-limiting condition. Babies who have trisomy 18 have a limited life expectancy, but many parents are choosing not to terminate the pregnancy, but to continue the pregnancy and spend as much time with their baby as possible.
Trisomy 18 diagnosis
How do you know if you’re carrying a baby with trisomy 18? Many people who have a baby affected with trisomy 18, don’t find out about it until the baby comes. We try to diagnose the condition prior to delivery if possible, to give you more of an opportunity to prepare for the special problems that these babies are going to have.
Just about all pregnant women are offered some kind of screening test for abnormal chromosomes. The most common chromosome abnormality is trisomy 21 (Down syndrome), and the second most common is trisomy 18.
Most screening tests include a screen for trisomy 21 and trisomy 18. For instance, the quad screen or penta screen is a blood test that looks at different proteins produced by the pregnancy. Depending on the combination of values, your risk of having a baby affected with trisomy 18 or trisomy 21 goes up or down. Another screening test that’s more accurate than the quad screen or the Penta screen is NIPT, or noninvasive prenatal screening. This screen looks at the risk of having a pregnancy with trisomy 21, trisomy 18, and trisomy 13 (the third most common chromosome abnormality). There are also first trimester screening tests which involve blood work and early ultrasound.
NIPT is a blood draw, so it’s noninvasive for your baby. The test looks directly at fetal DNA in your circulation. It has a very high sensitivity for trisomy 13, trisomy 18, and 21, but it’s still just a screening test, not a diagnostic test. This means that it doesn’t give you a 100% yes or no answer. When you are thinking about having one of these tests, you need to be counseled about what the test will tell you and what it won’t. Some people have a test and get an abnormal result, when that might be information they didn’t want.
Chorionic villus sampling
The only way to diagnose trisomy 18 definitively prior to delivery is by looking at the baby’s chromosomes directly with chorionic villus sampling (CVS), which can be performed earlier in pregnancy, or amniocentesis, where we place a needle into the amniotic sac around the baby, take out a little fluid with fetal cells floating in it, and culture the fetal cells.
Both CVS and amniocentesis will give us a definitive diagnosis, a definite yes or no about whether your baby has trisomy 18 or another chromosomal abnormality. What you need to know about CVS and amniocentesis is that they are invasive procedures. That means there’s a chance of having a complication that might lead to a pregnancy loss.
With amniocentesis, the risk of complications is very low. For instance, if you go to a maternal-fetal medicine practice where we perform amniocentesis just about every day, the risk of having a complication is less than one in 300-500.
When complications happen, they include ruptured membranes or infection. Because we’re poking a hole into the sac around the baby, sometimes fluid can leak out of that hole. There’s a good chance, though, that the leak will reseal and the pregnancy can be carried to term.
The second complication can be an infection. We are really careful about sterile technique when we’re doing your amniocentesis, but there’s a small possibility that bacteria could be introduced into the amniotic sac. And after a few days, you might develop a fever or an infection inside the uterus. The only treatment for that is to deliver the baby.
So those are the two most common complications of amniocentesis, but the sum total of complications is still less than 1%.
I see a lot of people start shaking their heads as I’m describing an amniocentesis. They say no right away, even though it’s the best test if you want the most information about your baby. However, amniocentesis is not for everyone. Some people tell us after an abnormal blood test that they don’t want amniocentesis. We often hear women say, “I’m going to continue the pregnancy no matter what. I’m going to love my baby, no matter what.”
The reason most people have amniocentesis is to have a yes or no as to whether their baby has a chromosomal abnormality like trisomy 18, so they can move forward and prepare for what it’s like to have a baby with trisomy 18.
One question I’m asked pretty frequently is,” My ultrasound looks just fine, how can my baby have a chromosome abnormality?” or “Everything’s fine, right? Because the ultrasound looks good?”
A normal ultrasound can certainly reduce your risk of having a baby with a chromosome abnormality, but it doesn’t eliminate it. There are babies who have Down syndrome or trisomy 18 who may not have any abnormalities that we can see on ultrasound. Especially at 18 to 20 weeks, which is when most screening ultrasounds are performed.
Trisomy 18 ultrasound findings
Babies who have trisomy 18 may have major birth defects, but frequently the findings may be subtle. Their growth may lag behind. They may have low set ears. They may have clenched fingers or fingers that overlap. Sometimes they can have clubbed feet.
Sometimes they have birth defects like extra fluid in the head or open spine defects. But lots of babies with trisomy 18 don’t have any visible birth defects, especially when you have your early screening ultrasound. More findings may appear as you get closer to your due date. Babies with trisomy 18 may not have good muscle tone. They may not swallow amniotic fluid as typical babies do. And so amniotic fluid tends to build up. One of the signs that we can see on ultrasound is extra fluid around the baby. Sometimes they’ll lag behind in growth or sometimes a heart abnormality that may not have been detected at your earlier screening may become more obvious later in the pregnancy.
A normal ultrasound is always great, but you can’t count on a normal ultrasound to rule out genetic or chromosome abnormalities.
That pretty much sums up screening and testing for trisomy 18. Many people choose to not be tested at all. And that’s perfectly reasonable. It’s important to talk through what you would do if your test is reported as abnormal. And if the answer is “I’m not going to do anything except worry for the six months,” maybe that test is not for you. So be sure and talk through this with your provider.
If you want to know more about trisomy 18, leave me a comment below with anything you would like me to cover.
In the next episode, we’re going to talk about something that parents always ask me when they’re carrying a baby with trisomy 18, “Will my baby survive? What is the life expectancy for trisomy 18? What is the quality of life going to be like for my baby? And what can I expect?”
We’re gonna dive deeper into all of those questions in the next episode. Stay tuned.