After a fetal arachnoid cyst is discovered on ultrasound, one of the first questions parents ask is:
“Will it get bigger?”
And often right after that:
“Can it disappear before my baby is born?”
The answer to both questions is:
It can.
During pregnancy, an arachnoid cyst may increase in size, remain relatively stable, or occasionally become smaller or less visible.
But the change in size is only part of what matters.
The more important question is:
Is the cyst affecting the developing brain around it?
Why Do We Follow an Arachnoid Cyst Over Time?
A single ultrasound gives us a snapshot.
Follow-up ultrasound gives us a trend.
When an arachnoid cyst is identified, serial imaging may help determine whether it is:
growing
stable
or
becoming less prominent.
At the same time, the examiner reassesses the surrounding fetal brain.
That second part is crucial.
Can an Arachnoid Cyst Grow During Pregnancy?
Yes.
Some fetal arachnoid cysts can enlarge as pregnancy progresses.
Growth does not automatically mean that the baby will have a poor outcome.
However, a growing cyst deserves careful evaluation because a larger cyst may begin to exert mass effect on nearby structures.
That’s why doctors don’t simply record:
“The cyst is now ___ mm.”
They also ask:
“What effect is it having?”
What Is Mass Effect?
Mass effect means that a lesion is pushing on nearby structures.
With an arachnoid cyst, ultrasound or fetal MRI may show displacement or compression of nearby brain anatomy.
Depending on the cyst’s location, this could involve:
- cerebral hemispheres
- lateral ventricles
- midline structures
- corpus callosum
- cerebellum or posterior fossa structures
A relatively large cyst without significant mass effect may therefore be interpreted differently from a cyst that is distorting surrounding anatomy.
Can It Cause Ventriculomegaly?
In some cases, yes.
A cyst may compress nearby structures or interfere with normal cerebrospinal fluid pathways.
This can contribute to enlargement of the fetal ventricles.
That is why follow-up imaging often includes careful reassessment of the lateral ventricles.
But an arachnoid cyst does not automatically cause ventriculomegaly.
What If the Cyst Stays the Same Size?
Stability can be reassuring, particularly when:
the surrounding brain continues to develop normally
and
no additional abnormalities appear.
However, doctors still consider location and anatomy.
Even a stable cyst may need continued observation if it is large or positioned near important structures.
So:
stable size is useful information—but it isn’t the only information.
Can an Arachnoid Cyst Become Smaller?
Yes.
Some arachnoid cysts may become smaller during pregnancy.
Occasionally, a cyst that was clearly visible earlier may become much less prominent on later imaging.
This can be reassuring.
However, regression should still be interpreted in the context of the complete fetal brain examination.
Can It Completely Disappear?
Some prenatally identified cystic lesions may no longer be visible on later imaging.
But this should be interpreted carefully.
There are several possibilities:
true regression
change in appearance
difficulty visualizing the lesion because of fetal position
or
refinement of the original diagnosis after better imaging.
So if a cyst is no longer seen, the examiner still evaluates the surrounding brain rather than simply concluding:
“It’s gone, so we’re finished.”
Why Might the Diagnosis Change on Follow-Up?
Fetal brain imaging becomes more detailed as pregnancy progresses.
A cyst first detected on a screening ultrasound may initially be described broadly as an intracranial cyst.
Later neurosonography or fetal MRI may better define:
- whether it is truly extra-axial
- its exact location
- its relationship with nearby structures
- whether another type of cystic lesion is more likely
A more specific diagnosis later does not necessarily mean the first scan was wrong.
Sometimes the anatomy simply becomes clearer.
Does a Growing Cyst Mean My Baby Will Need Surgery?
Not automatically.
Prenatal growth alone does not determine whether treatment will be necessary after birth.
Some babies with arachnoid cysts may only need observation.
Others may require neurosurgical assessment if the cyst causes:
progressive mass effect
hydrocephalus
or
neurological problems after birth.
The decision about treatment depends on the baby’s postnatal imaging and clinical condition.
If It Disappears, Does That Guarantee Everything Is Normal?
No prenatal imaging finding can provide that guarantee.
Regression is generally reassuring, but clinicians still consider whether:
- the rest of the fetal brain is normal
- ventricular size is normal
- brain structures were displaced earlier
- additional abnormalities are present
- genetic evaluation was indicated
Again, the whole brain matters more than the cyst alone.
What Does Follow-Up Ultrasound Look For?
At a follow-up examination, the sonographer may reassess:
1. Cyst size
Has it increased, decreased, or remained stable?
2. Cyst location
Is its relationship with surrounding structures clearer?
3. Mass effect
Is the cyst pushing on the developing brain?
4. Ventricular size
Has ventriculomegaly developed?
5. Midline structures
Are they appropriately positioned?
6. Remaining brain anatomy
Is development otherwise reassuring?
7. Additional fetal findings
Has anything else become apparent?
This is why follow-up is about much more than measuring the cyst again.
When Might Fetal MRI Be Helpful?
Fetal MRI may be considered when more anatomical detail is needed.
It can sometimes better demonstrate:
- exact cyst location
- relationship to brain tissue
- degree of mass effect
- corpus callosum
- cortical development
- additional intracranial abnormalities
MRI can be particularly useful when ultrasound visualization is limited or when the cyst is large or anatomically complex.
How Often Will Ultrasound Be Repeated?
There is no single follow-up schedule appropriate for every fetal arachnoid cyst.
Timing depends on factors such as:
gestational age
cyst size
location
whether it appears to be growing
presence of mass effect
and
associated abnormalities.
Your maternal-fetal medicine team will determine the appropriate interval for your pregnancy.
Does Size Predict Developmental Outcome?
Not by itself.
A measurement is useful for monitoring change, but prognosis depends on more than millimeters.
A better framework is:
size + location + growth + mass effect + associated findings.
For example, a cyst that grows but causes little distortion may represent a different situation from one affecting ventricular drainage or important brain structures.
What If the Cyst Is Isolated?
When an arachnoid cyst remains isolated and the surrounding fetal brain develops normally, the outlook can be favorable.
However, prenatal imaging cannot predict every aspect of future neurological development.
Postnatal follow-up may therefore still be recommended depending on the cyst.
Will My Baby Need Imaging After Birth?
Often, postnatal imaging may be considered to confirm:
- whether the cyst is still present
- its exact size and location
- whether it affects surrounding structures
- whether treatment or continued observation is needed
The exact plan depends on the prenatal findings.
Importantly:
postnatal imaging does not mean surgery is inevitable.
Many cysts can simply be monitored.
What Should I Ask at My Follow-Up Scan?
Instead of asking only:
“How big is it now?”
consider asking:
- Has the cyst grown?
- Is it stable or smaller?
- Is it still clearly an arachnoid cyst?
- Is it causing mass effect?
- Are the ventricles normal?
- Is the midline still in the expected position?
- Does the corpus callosum look reassuring?
- Is the rest of the fetal brain developing normally?
- Is the finding still isolated?
- Would fetal MRI add useful information?
- What imaging will my baby need after birth?
The most useful question may be:
“Has anything changed in the brain around the cyst?”
That often matters more than a few millimeters of change in the cyst itself.
Key Takeaway
A fetal arachnoid cyst can:
grow
remain stable
or sometimes
become smaller or less visible before birth.
But:
Growth does not automatically mean a poor outcome.
Persistence does not automatically mean surgery.
Regression does not replace evaluation of the rest of the brain.
What matters most is:
location + change over time + mass effect + ventricular size + associated findings.
So at the follow-up ultrasound, don’t focus only on:
“Is the cyst bigger?”
Ask:
“How is my baby’s brain developing around it?” 🧠👶
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. Follow-up ultrasound is valuable because fetal findings are dynamic. With an arachnoid cyst, measuring the cyst is only one part of the examination—the relationship between the cyst and the developing brain is often much more informative.
This article is for general educational purposes and does not replace individualized prenatal diagnosis, genetic counseling, or medical advice from your healthcare provider.
