Choroid plexus cysts (CPC) are fluid-filled sacs the develop brain's choroid plexus tissue. This tissue is responsible for producing cerebrospinal fluid (CSF) which protects the brain and spinal cord. These can be seen on 1-3 % of all babies on a second trimester (between 16 and 24 weeks) ultrasound during pregnancy. Generally, these cysts go away on their own during pregnancy, usually by 28 weeks, and without harming the baby. Although the exact cause of these cysts is unknown, they are considered a marker indication an increased risk for a chromosome abnormality called trisomy 18 (please also see: trisomy 18) which is a result of having an extra copy of chromosome 18. Although the risk is increased, it is still quite low.
If a choroid plexus cyst is found on ultrasound, additional testing is available including blood tests and an amniocentesis which looks at both the structure and number of the baby's chromosomes. Although these cysts typically resolve or disappear, the risk for a chromosome abnormality remains increased. It is important to remember that most of babies with a CPC seen on ultrasound are born healthy. If a CPC is detected during an ultrasound of your pregnancy, talk with your obstetrician or midwife. It is important to monitor the condition and understand all of your options so you can make the most informed choices regarding additional testing. Talking with a genetic counselor may also help you understand your available options clearly.