When a prenatal ultrasound unexpectedly reveals a choroid plexus cyst (CPC) during the second-trimester anatomy scan, it understandably causes parental concern. This small, fluid-filled pocket in the fetal brain is common, detected in approximately one to two percent of all pregnancies. While the discovery raises questions about underlying genetic conditions, the vast majority of these cysts are temporary and benign. Understanding the nature of the cyst and its clinical significance helps assess the risk and navigate the next steps in monitoring the pregnancy.
Understanding Choroid Plexus Cysts
The choroid plexus is a normal, sponge-like structure located within the fluid-filled spaces, or ventricles, of the brain. Its primary function is the production of cerebrospinal fluid, which nourishes and cushions the brain and spinal cord. A choroid plexus cyst is a tiny bubble of fluid trapped within this structure as the brain develops.
These cysts are defined on an ultrasound as cystic areas measuring over two millimeters in diameter. A CPC is not considered a structural brain abnormality and does not cause neurological damage or affect brain development. In most cases, these transient findings spontaneously resolve by the 26th to 28th week of gestation. Even if a cyst persists, it carries no implications for the baby’s health, provided no other abnormalities are present.
The Statistical Link to Chromosomal Conditions
Choroid plexus cysts are classified as a “soft marker” for aneuploidy, meaning they are a minor finding associated with an increased, though still low, statistical risk of a chromosomal condition. The strongest association is with Trisomy 18 (Edward Syndrome), which results from an extra copy of chromosome 18. Approximately 30 percent of fetuses with Trisomy 18 have a CPC detected on a prenatal ultrasound.
The statistical link to Trisomy 21 (Down Syndrome) is much weaker, and some studies suggest it is not significantly greater than the background risk. When an isolated CPC is found, the likelihood ratio for Trisomy 18 is substantially increased, potentially by a factor greater than seven, which guides further risk assessment. However, because Trisomy 18 itself is rare, affecting about one in 3,000 newborns, the overwhelming majority of fetuses with a CPC are chromosomally normal.
Differentiating Isolated and Non-Isolated Findings
The most important distinction in assessing the significance of a choroid plexus cyst is whether it is considered isolated or non-isolated. An isolated CPC is one where the cyst is the only unusual finding detected during a thorough second-trimester ultrasound examination. When a CPC is isolated, the risk of a major chromosomal abnormality is extremely low, often reported as zero percent in large studies of unselected populations.
A non-isolated CPC is found alongside one or more other structural abnormalities or soft markers. The presence of a CPC combined with other major markers, such as heart defects, restricted growth, or kidney abnormalities, significantly elevates the risk for Trisomy 18. This combination of findings prompts clinicians to recommend further diagnostic testing for a serious underlying condition.
Necessary Follow-Up and Monitoring
Once a choroid plexus cyst is identified, the immediate step is to conduct a highly detailed, targeted ultrasound. This examination confirms whether the finding is truly isolated by ruling out other structural markers associated with Trisomy 18 or Trisomy 21. If the CPC is isolated, further invasive diagnostic testing is often not recommended. This is because the risk of a procedure-related complication, such as pregnancy loss from an amniocentesis (around 1 in 200), may outweigh the risk of a chromosomal condition.
Non-invasive prenatal testing (NIPT), which screens for Trisomy 21 and 18 using cell-free DNA in the mother’s blood, is a common next step to refine the risk assessment. If a CPC is non-isolated or if the NIPT results indicate a high risk, genetic counseling is offered to discuss diagnostic options like amniocentesis for a definitive karyotype. For an isolated CPC in a low-risk pregnancy, many practitioners forgo additional ultrasounds, as the cyst is expected to resolve naturally.