A choroid plexus cyst (CPC) is a small, fluid-filled pocket often detected in the developing brain of a fetus during a routine ultrasound examination. These cysts are a common finding in prenatal screening, appearing in approximately one to two percent of all pregnancies scanned in the second trimester. The CPC is considered a “soft marker,” meaning it is a minor finding that usually does not harm the baby. These cysts typically resolve spontaneously and do not cause any long-term health or developmental problems.
What Exactly is a Choroid Plexus Cyst
The choroid plexus is a normal structure located within the ventricles, the fluid-filled spaces deep inside the brain. Its primary function is to produce cerebrospinal fluid (CSF), a clear liquid that bathes the brain and spinal cord to provide cushioning and protection.
A choroid plexus cyst is a small, encapsulated space filled with CSF that develops within the tissue of the choroid plexus. These cysts are typically identified during the detailed anatomy ultrasound performed between 16 and 24 weeks of gestation. They are not considered a brain abnormality or a tumor, but rather a temporary variation in the tissue’s development. The cysts can be unilateral, appearing on one side of the brain, or bilateral, appearing on both sides.
Understanding the Formation of Choroid Plexus Cysts
The definitive cause of choroid plexus cysts is not fully understood, but their formation is linked to the rapid development of the choroid plexus tissue early in pregnancy. The prevailing hypothesis suggests that a CPC forms when cerebrospinal fluid becomes temporarily trapped within the folds or layers of the developing choroid plexus cells. This process is often compared to the formation of a small blister.
The cysts are thought to be a transient developmental variation occurring during the formation of the fetal brain, starting around six weeks of gestation. This event is not caused by external or environmental factors. In most cases, the cyst is an isolated finding, meaning it occurs without any other structural anomalies in the fetus.
The Link Between Cysts and Chromosomal Conditions
A choroid plexus cyst is usually an isolated and harmless finding, but its detection is relevant due to a statistical association with certain chromosomal conditions, particularly Trisomy 18 (Edwards syndrome). Trisomy 18 is a serious genetic disorder caused by an extra copy of chromosome 18, leading to severe intellectual disability and multiple birth defects. CPCs are observed in 30 to 50 percent of fetuses diagnosed with Trisomy 18.
Isolated vs. Non-Isolated Findings
The distinction between an isolated CPC and a non-isolated CPC is important for risk assessment. An isolated choroid plexus cyst means a detailed ultrasound finds no other structural abnormalities or soft markers in the fetus. When the cyst is truly isolated, the risk of Trisomy 18 is very low, generally less than one percent, especially for women under 35.
The risk increases significantly if the CPC is non-isolated, meaning other anomalies are present, such as heart defects, clenched fists, or restricted growth. In these cases, healthcare providers may recommend further testing, such as non-invasive prenatal screening (NIPS) or diagnostic procedures like amniocentesis. These tests determine the fetal karyotype, which confirms or rules out a chromosomal condition. The size or number of the cysts does not change the risk when the finding is isolated; the presence of other markers warrants genetic investigation.
Monitoring and Outcome After Diagnosis
Management following the discovery of a CPC involves a detailed follow-up ultrasound, often called a Level II or targeted scan. This comprehensive scan ensures the cyst is an isolated finding and checks for other subtle markers associated with Trisomy 18. If the cyst is isolated and previous genetic screening results were reassuring, no further testing is usually necessary.
The prognosis for an isolated choroid plexus cyst is overwhelmingly positive, as the vast majority disappear completely on their own. The cysts usually resolve by the third trimester, often before 28 to 32 weeks of gestation, as the developing brain grows and the trapped fluid is reabsorbed. Even if the cyst persists until birth, it poses no known risk to the baby’s neurological development or intelligence.
Since the cysts are temporary and asymptomatic, no treatment is required for the CPC itself. The long-term outcome for children who had an isolated CPC detected prenatally is the same as for children who never had one. Once the cyst is confirmed to be isolated and resolves, the pregnancy is treated as a normal, low-risk pregnancy.