During a detailed fetal ultrasound, the two lungs usually have a fairly similar appearance.
So when one side of the chest suddenly looks:
brighter, larger, or different from the opposite lung,
it catches my attention.
Sometimes the report may use a broad term such as:
fetal lung lesion
or
fetal chest mass.
Parents naturally want a name immediately.
Is it CPAM? Is it a tumor? Will my baby be able to breathe?
But when I first see an abnormal area in the fetal chest, naming it isn’t my first job.
My first job is to understand:
what it is doing inside the chest.
First: Is It Really Coming From the Lung?
The fetal chest contains several structures packed into a small space:
- right and left lungs
- heart
- diaphragm
- mediastinum
- major blood vessels
And abdominal organs sit immediately below the diaphragm.
So before calling something a lung lesion, I confirm:
where the diaphragm is,
whether the stomach and liver are below it,
and
whether the abnormality truly lies within the fetal thorax.
This helps distinguish a lung lesion from conditions such as congenital diaphragmatic hernia.
What Can a Fetal Lung Lesion Look Like?
Not every congenital lung lesion looks like a round mass.
Some appear:
solid and echogenic.
Others contain:
multiple cystic spaces.
And some have a combination of solid-appearing and cystic components.
The appearance can also change as pregnancy progresses.
That’s why one frozen image rarely tells the whole story.
What Is CPAM?
One of the better-known congenital fetal lung abnormalities is:
Congenital Pulmonary Airway Malformation (CPAM).
CPAM involves abnormal development of part of the fetal lung.
On prenatal ultrasound, it may appear as:
- a predominantly echogenic lung lesion
- a lesion containing visible cysts
- or a mixture of the two
Older terminology may call it:
CCAM — congenital cystic adenomatoid malformation.
CPAM is the term more commonly used today.
Does CPAM Always Look Cystic?
No.
This surprises many parents.
The word “cystic” is historically associated with these lesions, but very small cysts may be below the resolution of ultrasound.
As a result, some CPAMs look almost:
solid and bright
rather than obviously cystic.
So the absence of large visible cysts does not automatically exclude CPAM.
What Is Bronchopulmonary Sequestration?
Another congenital lung lesion is:
bronchopulmonary sequestration.
This is abnormal lung tissue that does not have the usual connection with the tracheobronchial tree.
One of its most useful prenatal clues is its:
systemic arterial blood supply.
Instead of receiving its arterial supply in the usual pulmonary pattern, a sequestration may receive a feeding artery from the systemic circulation, often arising from the aorta.
And that is where Doppler becomes especially useful.
Why Does Color Doppler Matter?
If I see a solid-appearing echogenic lesion in the fetal chest, I look for:
a feeding vessel.
Color Doppler may demonstrate an artery traveling from the aorta toward the lesion.
That finding can strongly support the diagnosis of:
bronchopulmonary sequestration.
This is a perfect example of why grayscale appearance alone isn’t always enough.
Sometimes the most important clue isn’t the mass.
It’s the vessel feeding it.
Can CPAM and Sequestration Occur Together?
Yes.
Hybrid lesions with features of both CPAM and pulmonary sequestration have been described.
So prenatal lung lesions do not always fit neatly into one textbook category.
That is another reason reports may initially use a broader descriptive term rather than forcing an exact diagnosis too early.
The Question I Care About Most: Is the Heart Being Pushed?
The fetal chest has limited space.
As a lung lesion becomes larger, it can push the:
heart and mediastinum toward the opposite side.
This is called:
mediastinal shift.
A small lesion with the heart in a normal position is a very different situation from a large lesion occupying much of one hemithorax and markedly displacing the heart.
So when parents ask:
“How big is the mass?”
I also want to know:
“What effect is it having?”
Why Is the Diaphragm Important?
A large thoracic lesion may push downward on the diaphragm.
I check whether the diaphragm maintains its expected contour and whether abdominal organs remain in their normal positions.
This helps assess mass effect and also helps distinguish a primary lung lesion from:
congenital diaphragmatic hernia.
What Is CVR?
When CPAM is suspected, specialists may calculate the:
CPAM Volume Ratio (CVR).
This uses measurements of the lesion relative to fetal head circumference to provide a standardized way of following lesion size.
CVR can help estimate the risk of complications such as hydrops, particularly when considered alongside the entire ultrasound picture.
But:
CVR is not a crystal ball.
One value cannot perfectly predict outcome.
The trend and the baby’s condition matter.
Why Are Serial Ultrasounds Important?
Congenital lung lesions can change substantially during pregnancy.
Some grow rapidly during part of the second trimester.
Later, many appear to:
plateau
or even become less conspicuous relative to the growing lungs.
So when we follow a lesion, we are not merely repeating the same scan.
We are asking:
Is it growing?
Is the heart shifting?
Is hydrops developing?
Is the fetus otherwise doing well?
The direction matters.
Can a Lung Mass “Disappear” Before Birth?
Sometimes a lesion becomes much harder to see later in pregnancy.
That does not necessarily mean the abnormal lung tissue has completely disappeared.
The echogenicity of the lesion can become more similar to surrounding normal lung, making it less conspicuous on prenatal ultrasound.
Therefore, even if a previously identified lesion becomes difficult to visualize:
postnatal imaging may still be recommended.
This is an important point for parents.
“Can’t see it anymore” does not always mean:
“It never existed.”
Why Do We Look for Hydrops?
Large thoracic lesions can impair venous return and place stress on the fetal cardiovascular system.
In severe cases, this can contribute to:
fetal hydrops.
I therefore check for findings such as:
- ascites
- skin edema
- pleural effusion
- pericardial effusion
as well as cardiac position and function.
Hydrops dramatically changes the significance of a fetal lung lesion.
Can a Lung Lesion Cause Polyhydramnios?
It can be associated with increased amniotic fluid in some cases.
A large thoracic mass may interfere with fetal swallowing through compression or altered thoracic anatomy.
So amniotic fluid is another part of the surveillance examination.
But polyhydramnios alone does not identify the type of lung lesion.
Does a Fetal Lung Mass Mean Lung Cancer?
No.
CPAM and pulmonary sequestration are congenital developmental lung abnormalities.
They are not the same thing as an adult lung cancer.
Using the word “mass” on an ultrasound report can sound frightening, but in imaging:
mass simply describes an abnormal space-occupying area.
It does not automatically mean malignancy.
Can the Normal Lung Still Develop?
Often, yes.
The amount of normal lung development depends on factors such as:
- lesion size
- timing
- degree of compression
- whether one or both sides are involved
- associated abnormalities
- presence or absence of hydrops
A small unilateral lesion may leave substantial normal lung tissue.
A very large lesion producing prolonged compression deserves much closer surveillance.
What Else Does the Sonographer Check?
Once I identify a possible fetal lung lesion, I stop thinking about it as an isolated “spot.”
I check:
Both lungs
How much normal lung is visible?
Heart
Is it in the expected position?
Mediastinum
Is there a shift?
Diaphragm
Is it intact and normally positioned?
Feeding vessels
Is there systemic arterial supply?
Pleural spaces
Any fluid?
Abdomen
Any ascites?
Skin
Any edema?
Amniotic fluid
Normal or increased?
Fetal growth and anatomy
Anything else abnormal?
That is the real examination.
Could Fetal MRI Help?
In selected cases, yes.
Fetal MRI can provide additional information about:
lesion extent
remaining lung tissue
thoracic anatomy
diaphragm
and the relationship between the lesion and surrounding structures.
MRI complements ultrasound.
It doesn’t make ultrasound unnecessary.
Can Fetal Lung Lesions Be Treated Before Birth?
Most do not require prenatal intervention.
But severe cases complicated by hydrops or significant mass effect may require evaluation at a specialized fetal therapy center.
Management depends heavily on the type of lesion and its features.
Selected treatments can include maternal medication for certain CPAMs, drainage or shunting for appropriate large cystic lesions, and rarely other fetal interventions.
These treatments are not routine for every fetal lung lesion.
Will My Baby Need Surgery After Birth?
Some babies with congenital lung lesions eventually undergo surgical removal of the abnormal lung tissue.
Others may be managed differently depending on:
- symptoms
- lesion type
- postnatal imaging
- size
- local pediatric surgical practice
A baby who looks completely well after birth may still undergo imaging because prenatal ultrasound cannot always define the final anatomy.
Does This Mean I Need a C-Section?
Not automatically.
A fetal lung lesion alone does not necessarily require cesarean delivery.
Delivery planning depends on:
- lesion severity
- hydrops
- anticipated respiratory needs
- obstetric factors
- available neonatal resources
For significant lesions, where the baby is delivered may be more important than assuming a particular route of delivery.
Questions to Ask Your Doctor
If a fetal lung lesion is found, useful questions include:
- Is the lesion cystic, solid-appearing, or mixed?
- Is CPAM suspected?
- Could this be pulmonary sequestration?
- Is there a systemic feeding vessel?
- Is the heart shifted?
- How much normal lung can be seen?
- Is there any sign of hydrops?
- Is the lesion growing?
- Are you following a CVR?
- Is the amniotic fluid normal?
- Would fetal MRI add useful information?
- How often will ultrasound be repeated?
- Should I meet a pediatric surgeon before delivery?
- Will my baby need imaging after birth?
- Does delivery need to occur at a tertiary center?
Those questions tell you far more than:
“How many centimeters is the mass?”
Key Takeaway
A fetal lung mass is not one single diagnosis.
The ultrasound assessment is a process.
We look at:
appearance
blood supply
growth
heart displacement
normal remaining lung
and
whether hydrops is developing.
When I find an abnormal bright area in a fetal lung, I certainly measure it.
But then I put the calipers down.
And I look at the heart.
I look at the diaphragm.
I turn on Doppler and search for a feeding vessel.
I look for fluid elsewhere.
Because with congenital lung lesions:
the mass has a name—but its behavior tells us why it matters.
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.
When a fetal lung looks different from the opposite side, I don’t try to force a diagnosis from the grayscale appearance alone. Cardiac displacement, lesion evolution, remaining lung tissue, hydrops, and especially the vascular supply on Doppler can be much more informative in distinguishing and following congenital thoracic lesions.
This article is for general educational purposes and does not replace individualized prenatal diagnosis, maternal-fetal medicine evaluation, fetal MRI, fetal therapy consultation, pediatric surgical evaluation, or medical advice from your healthcare provider.
