A choroid plexus cyst is a small, fluid-filled space that occurs in a gland in the brain called the choroid plexus. This gland is located on both the left and right sides of the brain, and the fluid it produces surrounds the brain and spinal cord to protect them from injury.

This type of cyst develops in some fetuses. A doctor will usually see it on an ultrasound scan during a pregnant person’s second trimester.

It occurs in about 1 to 2 percent of pregnancies. It generally poses no problems and resolves on its own before birth. These cysts are seen equally in males and females.

For reasons that aren’t fully understood, a choroid plexus cyst can form when fluid becomes trapped within the layers of cells of the choroid plexus. There can be one or several.

The formation of this type of cyst can be likened to a blister that forms below the skin. The choroid plexus begins developing at about 6 weeks of gestation. By around 25 weeks, a choroid plexus cyst can be visible on an ultrasound.

The choroid plexus isn’t in an area of the brain involved in thinking, deducing, or reasoning. It has no impact on a person’s intelligence or cognitive development.

When seen by itself, with all other systems developing normally, a choroid plexus cyst is what scientists call a normal variant. It produces no health or intellectual disorders or disabilities.

Choroid plexus cyst and trisomy 18

Choroid plexus cysts develop about a third of the time in fetuses with trisomy 18. Trisomy 18, also called Edwards syndrome, is a condition in which a fetus has three copies of chromosome 18 instead of two.

This extra chromosome, which the embryo can inherit from a parent or develop randomly at conception, wreaks havoc on the human body.

Many fetuses with trisomy 18 don’t survive to birth because of organ abnormalities. Babies born with the condition tend to have significant birth defects, including:

Only about 10 percent of babies born with trisomy 18 live past their first birthday, and they often have severe mental disabilities. According to the University of California San Francisco (UCSF) Medical Center, trisomy 18 is rare, occurring in just 1 in 3,000 babies.

While many fetuses with trisomy 18 also have choroid cysts, only a small percentage of those with a choroid plexus cyst will also have trisomy 18.

A fetus with trisomy 18 will have other abnormalities seen on an ultrasound besides the choroid plexus cyst. If your doctor suspects other abnormalities that suggest trisomy 18, they may recommend the following tests to help confirm the diagnosis:

  • Alpha-fetoprotein test. Some things, like a miscalculated due date or having twins, can affect the results of an alpha-fetoprotein (AFP) test. The test also tends to provide a high number of false positives, meaning the test may indicate your fetus has an abnormality when it doesn’t. An AFP test requires a simple blood draw and poses little to no risk to you or the fetus.
  • Level 2 ultrasound. This sophisticated ultrasound carries no health risks and can give detailed, comprehensive views of the anatomy of a fetus. The view it provides may allow your doctor to see some anatomical abnormalities, such as clenched fists and a small mouth, indicative of trisomy 18.
  • Amniocentesis. An amniocentesis is a test that uses an ultrasound-guided needle to draw out amniotic fluid to test for birth defects and genetic conditions. The fluid is sent to a lab where cells are extracted and examined. Miscarriage is a risk of amniocentesis, but fewer than 1 percent of women who undergo the procedure will lose their baby before it’s born.

A choroid plexus cyst is something that doctors may discover incidentally during a routine middle-of-pregnancy ultrasound.

Picture of choroid plexus cyst

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Nevit Dilmen/Wikimedia Commons

This type of cyst typically requires no treatment, as 90 percent will resolve on their own by the 28th week of gestation. Even when an otherwise healthy child is born with a choroid plexus cyst, the baby will likely develop normally.

Surgery to remove the cyst is usually reserved for rare cases when the testing determines that the cyst is actually a cancerous tumor known as choroid plexus carcinoma (CPC).

Any abnormal test result can add to the uncertainty of pregnancy, but the finding of a choroid plexus cyst, especially in an otherwise normally developing fetus, is not usually a cause for alarm.

In a third of cases, the cysts may develop in babies with trisomy 18, which causes a variety of neurological and developmental problems.

Most of the time, however, the cysts pose no health concerns, and most babies with these cysts are born healthy and develop normally.