A choroid plexus cyst is a cyst that can grow in the brain of a fetus during development. They are most commonly seen in the second and third trimester of the pregnancy and can be detected via an ultrasound. The appearance of a choroid plexus cyst generally is not a cause for alarm, but these cysts sometimes indicate an increased risk of Down syndrome.The choroid plexus of the brain is a small area near the lower middle portion of the organ that produces cerebral spinal fluid. This fluid surrounds the brain and spinal cord, acting as a physical and immunological buffer to protect these structures from injury, infection and inflammation. In the brain, this fluid is essential for another reason: it provides the brain with a measure of buoyancy, which allows it to bear its own weight.AdWithin the choroid plexus are layers of blood vessels and fluid-producing cells. As fluid is produced, it flows from the choroid plexus and circulates around the organ and the spinal cord. In a developing fetus, cysts can develop in the choroid plexus when fluid is trapped within layers of cells. It is also thought that adults often have one or more cysts. In both fetuses and adults, the cysts are harmless and do not affect physical or cognitive development, learning capacity or health in any way. In a developing fetus, a choroid plexus cyst is almost always temporary and disappears during the third trimester.Choroid plexus cysts are relatively common. About one percent of fetuses in the second and third trimester have a choroid plexus cyst that can be detected via an ultrasound. Even though the cysts themselves are harmless, there is a link between the appearance of these cysts and certain genetic disorders.If a fetus has a choroid plexus cyst and the pregnant woman has certain other risk factors, she generally will be recommended for amniocentesis. In this test, a small sample of amniotic fluid is removed from the uterus. Fetal cells can be removed from this fluid and tested for genetic disorders such as Down syndrome and Trisomy 18, a disorder in which a fetus has three copies of chromosome 18. The main risk factor for both of these genetic disorders is the age of the pregnant woman, with the risk increasing as a woman gets older. There also is evidence to suggest that increasing the age of the father also is a risk factor.