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 1-3% of all pregnancies scanned between 16 and 24 weeks gestation. In the majority of cases, CPCs disappear by the 28th week of pregnancy with no effect on the baby. However, a fetal CPC is considered a "marker", indicating that the baby may have an increased risk for a chromosome abnormality. When observed as an isolated ultrasound finding in women under 35 years of age, the risk for trisomy 18 is increased, but remains well within the normal range. 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. Chromosomes are the inherited structures in the cells of the body. There should be 46 chromosomes in each cell, arranged into 23 pairs. Chromosome abnormalities involving an entire missing or extra chromosome are not inherited and are not caused by an exposure during pregnancy. Instead, they are caused by random mistakes in cell division at the time of conception and can occur in anyone's pregnancy. Infants with trisomy 18 have severe mental retardation 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. 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. Maternal serum quad screening, if not previously performed, may be considered to screen the pregnancy for trisomy 18, as well as Down syndrome and open neural tube defects. Amniocentesis, to test for chromosome abnormalities in the baby, is also an option.
It is important to remember that isolated CPCs are usually normal variants that have no negative effect on the baby.