A choroid plexus cyst (CPC) is found during your baby’s anatomy scan.
Then you search online and see:
“CPC can be associated with trisomy 18.”
That sentence can turn a relatively common ultrasound finding into something terrifying.
But there is an important distinction:
A choroid plexus cyst is not a diagnosis of trisomy 18.
Its significance depends heavily on whether the CPC is isolated, what the rest of the anatomy scan shows, and what prenatal aneuploidy screening has already shown.
Why Is CPC Associated With Trisomy 18?
Choroid plexus cysts have historically been recognized as an ultrasound soft marker associated with trisomy 18, also called Edwards syndrome.
A soft marker is an ultrasound finding that can occur in healthy fetuses but may also be seen more frequently with certain chromosomal conditions.
That means:
CPC = soft marker
not:
CPC = chromosomal abnormality.
This difference is essential.
What Is Trisomy 18?
Trisomy 18 occurs when there is an extra copy of chromosome 18.
Fetuses with trisomy 18 frequently have multiple structural abnormalities that may be detectable during detailed ultrasound.
These can involve areas such as the:
- heart
- brain
- hands and feet
- face
- abdominal wall
- fetal growth
The exact findings vary from fetus to fetus.
This is why the entire anatomy scan matters much more than the CPC alone.
Why Does “Isolated CPC” Matter So Much?
Suppose the ultrasound shows a CPC but:
the heart looks reassuring
the brain otherwise looks reassuring
the hands and feet appear normal
growth is appropriate
and
no additional structural abnormality or relevant marker is identified.
This is an isolated CPC.
That is a very different clinical situation from a fetus with a CPC plus multiple abnormalities suggestive of trisomy 18.
Does an Isolated CPC Mean My Baby Has Trisomy 18?
No.
An isolated CPC alone does not diagnose trisomy 18.
When interpreting the finding, clinicians consider the baby’s complete anatomy and any previous prenatal screening.
This is particularly important today because screening methods such as cell-free DNA/NIPT provide much more useful information about trisomy 18 risk than an isolated CPC does.
My NIPT Was Low Risk. Should I Still Worry?
This is one of the most common questions.
If your baby has an isolated CPC and you already have a negative/low-risk serum screening or cell-free DNA result, current SMFM guidance considers this a normal variant with no clinical importance.
In that situation, SMFM recommends no further aneuploidy evaluation solely because of the isolated CPC, and no follow-up ultrasound or postnatal evaluation is required just for the CPC.
That is an important distinction from situations where additional abnormalities are present.
What If I Haven’t Had Prenatal Genetic Screening?
If an isolated CPC is found and you have not previously had aneuploidy screening, your healthcare provider may discuss screening options.
These may include:
cell-free DNA/NIPT
or
serum screening, depending on availability and your individual situation.
The purpose is to assess the pregnancy’s trisomy 18 risk more accurately—not because the CPC itself proves anything.
Does CPC Size Change the Trisomy 18 Risk?
Parents sometimes focus heavily on the measurement:
“Mine is 4 mm.”
“Mine is 7 mm.”
“There are two cysts.”
But CPC size alone should not be used as a simple predictor of trisomy 18.
Similarly, whether the CPC is:
unilateral
or
bilateral
does not turn it into a genetic diagnosis.
The overall fetal evaluation and screening history matter far more.
What If the CPC Disappears?
Most CPCs resolve spontaneously later in pregnancy.
But here’s an important concept:
The cyst disappearing does not change a chromosome result.
Likewise, a CPC remaining visible does not mean the baby has trisomy 18.
The cyst’s appearance over time and the baby’s chromosome status are two different issues.
Why Isn’t Another Ultrasound Always Necessary?
Parents understandably want to see that the cyst has disappeared.
But when an isolated CPC is found after negative aneuploidy screening, professional guidance does not recommend another ultrasound solely to check whether the CPC has gone away.
Why?
Because disappearance does not provide meaningful additional information about chromosome 18.
Another ultrasound may still be performed for other pregnancy-related reasons, of course.
What If Other Abnormalities Are Found?
This changes the situation.
If a CPC is accompanied by other structural abnormalities or concerning ultrasound findings, clinicians do not treat it as a simple isolated soft marker.
The entire pattern needs to be evaluated.
Depending on the findings, your healthcare team may discuss:
- detailed fetal ultrasound
- maternal-fetal medicine consultation
- genetic counseling
- additional screening
- diagnostic testing such as amniocentesis
The recommendation depends on the whole clinical picture.
Is NIPT the Same as Amniocentesis?
No.
This distinction is important.
NIPT/cell-free DNA is a screening test.
It estimates the likelihood of certain chromosomal conditions with high accuracy, but it is not diagnostic.
Amniocentesis is a diagnostic procedure that can directly evaluate fetal genetic material.
Most pregnancies with an isolated CPC and reassuring prior screening do not suddenly require amniocentesis solely because the CPC was found.
Does CPC Affect Brain Development?
An isolated CPC itself does not damage the fetal brain.
This is another reason the term can be misleading.
The concern historically associated with CPC is about its role as a soft marker, not because the cyst is harming brain tissue.
So:
CPC does not need to be removed.
CPC does not need treatment.
And most disappear naturally.
What Should I Ask My Doctor?
If your baby’s CPC has you worried about trisomy 18, ask:
- Is the CPC truly isolated?
- Was the entire anatomy scan completed?
- Were any structural abnormalities identified?
- Were any other soft markers seen?
- What were my previous prenatal screening results?
- Was my trisomy 18 screening low risk?
- Does this CPC meaningfully change my estimated risk?
- Do I need additional screening?
- Is diagnostic testing actually indicated?
- Do I need another ultrasound specifically for the CPC?
And perhaps the most useful question:
“If my screening is low risk and the CPC is isolated, does this finding change my pregnancy management?”
For many families, that answer is very reassuring.
Key Takeaway
Yes, choroid plexus cysts have historically been associated with trisomy 18.
But that statement needs context.
CPC is a soft marker—not a diagnosis.
An isolated CPC does not mean your baby has trisomy 18.
The complete anatomy scan matters.
And when an isolated CPC is found after negative serum or cell-free DNA screening, current SMFM guidance recommends no further aneuploidy evaluation solely because of the CPC.
So instead of searching:
“CPC = trisomy 18?”
the more useful questions are:
“Is it isolated?”
and
“What did my prenatal screening show?” 👶🧬
About the Author
This article was written by a sonographer with over 20 years of hands-on clinical ultrasound experience, including fetal, breast, and thyroid imaging. CPC is one of those findings where the word cyst and its historical association with trisomy 18 can create far more anxiety than the finding itself warrants. My goal is to help parents understand how we interpret a soft marker in the context of the complete anatomy scan and modern prenatal screening.
This article is for general educational purposes and does not replace individualized prenatal diagnosis, genetic counseling, or medical advice from your healthcare provider.
