Choroid Plexus Cyst - Prenatal Genetics

The second trimester ultrasound examination (sonogram) will sometimes identify a cyst or cysts in the choroid plexus. The choroid plexus is a tissue in the brain that produces cerebrospinal fluid. Fluid-filled cysts, called choroid plexus cysts or CPCs, are identified by ultrasound in approximately 1-2% of all pregnancies scanned between 16 and 24 weeks gestation. In almost all cases, CPCs disappear by the 28th week of pregnancy with no effect on the baby. CPC is not considered a brain abnormality. However, a fetal CPC is considered a "soft marker," indicating that the baby may have an increased risk for a chromosome abnormality, trisomy 18. When observed as an isolated ultrasound finding in women under 35 years of age, the risk for Trisomy 18 is increased, but remains low. The risk is higher for women age 35 or older. Additional abnormal ultrasound findings significantly increase the risk for Trisomy 18. CPCs do not increase the risk for Down syndrome in the pregnancy. 

Trisomy 18 is a chromosome abnormality resulting from an extra copy of chromosome #18, thus three copies instead of two. Infants with Trisomy 18 have severe intellectual disability and multiple birth defects. Many pregnancies with Trisomy 18 result in a miscarriage or infant death, although a small percentage can live for several years. 

When a CPC is identified on ultrasound, there is the option for additional testing. A more detailed ultrasound, to look for both major birth defects and minor findings associated with Trisomy 18, may be recommended, depending upon the amount of detail that was obtained during previous ultrasounds. Prenatal cell-free DNA screening (NIPS) is recommended if not yet performed. If the screening is reassuring (negative), no further diagnostic testing (amniocentesis) is recommended. Prenatal diagnostic testing by amniocentesis is, however, always available for any indication, to test for chromosome abnormalities during pregnancy. 

Genetic counseling to review the implications of choroid plexus cyst(s) is available. 

It is important to remember that isolated CPCs are usually normal variants that have no negative effect on the baby.