The brain contains pockets or spaces called ventricles with a spongy layer of cells and blood vessels called the choroid plexus. This is in the middle of the fetal brain. The choroid plexus has an important function of producing a fluid called cerebrospinal fluid. The fluid produced by the cells of the choroid plexus fills the ventricles and then flows around the brain and the spinal cord to provide a cushion of fluid around these structures. Choroid plexus cysts (CPCs) occur within this structure and come from fluid trapped within this spongy layer of cells, much like a soap bubble or a blister.

It is believed that many adults have one or more tiny CPCs. CPCs have no impact on an individual's health or development or learning. The fetal brain may create these cysts as a normal part of development. They are temporary and usually are gone by the 32nd week of pregnancy.

Chromosome problems

Genetic counseling is often recommended to provide more information about fetal CPCs, to answer questions and concerns, and to outline available options such as amniocentesis. There is a possible association between ultrasound-detected fetal CPCs and chromosome problems in the baby. Types of chromosome problems that are occasionally seen include Trisomy 18 or Trisomy 21 (Down syndrome).

Generally the risks are low if there are no other risk factors. The risk is estimated to be 1% (1/100) of delivering a baby with a chromosome problem when there is a CPC present.

Other factors which may have a bearing on the baby's chances of developing chromosome problems include:

• mother's age at the expected date of delivery
• the results of XAFP testing
• evidence of other "fetal findings" seen at the time of the ultrasound that may suggest a chromosome problem

Many babies with chromosome problems do not show any signs on ultrasound.

See also The Choroid Plexus Cyst Website (http://ChoroidPlexusCyst.org)