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 side of the brain, and it makes fluid within the brain and spinal cord. This fluid helps cushion both structures from injury.

A choroid plexus cyst is found in some fetuses and is usually picked up on an ultrasound during a woman’s second trimester of pregnancy. It occurs in about 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 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. It hasn’t been associated with any learning disabilities or spectrum disorders, such as autism.

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 are found in 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 may be inherited from a parent or happen randomly at conception, wreaks havoc on the human body.

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

Only 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 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 other abnormalities that suggest trisomy 18 are seen or suspected, your doctor 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 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 your developing baby.

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 of the anatomical abnormalities, such clenched fists and 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 of women who undergo the procedure will lose their baby before it’s born.

A choroid plexus cyst is an incidental finding discovered during a routine middle-of-pregnancy ultrasound.

Picture of choroid plexus cyst

A choroid plexus cyst, by itself, is a harmless collection of fluid that can sometimes be seen in the choroid plexus portion of the brain. That said, it can occur with more serious conditions, such as Edwards syndrome.

This type of cyst requires no treatment, as 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. In that looked at nearly 200 babies born with the cysts, the vast majority resolved by six months of age, and none of the babies had abnormal development.

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 a cause for alarm. The cysts often pose no health concerns, and most babies with these cysts will be born healthy and develop normally.