The Choroid Plexus and Choroid Plexus Cysts
The choroid plexus is an area of the brain that makes cerebrospinal fluid. Cerebrospinal fluid is the substance that surrounds the brain and spinal cord. The choroid plexus is not an area of the brain involved in thinking. Choroid plexus cysts are fluid-filled spaces in the choroid plexus kind of like blisters or bubbles. Choroid plexus cysts are not tumors or cancer, and they will disappear on their own usually between 24 to 26 weeks of pregnancy. Choroid plexus cysts, in and of themselves, are NOT a problem and do not interfere with brain function.
They are seen in about 1% (1/100) of all second trimester ultrasounds. Choroid plexus cysts may be seen in one or both sides of the brain and the number, size, and shape of the cysts may vary. Choroid plexus cysts are also found in healthy children and adults. Although they sounds alarming, they are not associated with any impairment in brain function. The bigger concern with choroid plexus cysts is that they may be a “soft marker” for Down syndrome. A “soft marker” is something that in and of itself, won’t cause your baby a problem, but it might indicate that the baby has a more serious underlying condition. Other soft markers for Down syndrome include a shortened femur measurement, renal pelvic dilatation, echogenic bowel, echogenic foci in the heart, an increased fetal nuchal translucency (the area at the back of the neck), and an absent nasal bone in the first trimester.
Ultrasound During Pregnancy and Choroid Plexus Cysts
An ultrasound is a test done during pregnancy. It uses sound waves to view or visualize the fetus during a pregnancy. No risk to the baby is associated with ultrasound during pregnancy and most women will have one or two ultrasounds, for a variety of reasons, over the course of their pregnancy.
It is most likely that the choroid plexus cysts are normal for you baby and you will have a normal, healthy child. However, some researchers have seen an association between these cysts and chromosome abnormalities like Down syndrome and trisomy 18. When choroid plexus cysts and/or other markers are seen on an ultrasound, you may be offered further testing such as a level 2 or detailed ultrasound, or an amniocentesis.
What is the chance that the Baby has Down Syndrome or Trisomy 18?
The chance that a fetus with isolated choroid plexus cyst(s) has a chromosome abnormality is felt to be about 1% or less, if the rest of the ultrasound exam was normal. This means that there is a 99% or greater chance that the baby does not have a chromosome abnormality. Some researchers have reported risks greater than 1% and yet others have seen no association between these cysts and chromosome abnormalities. It is best to discuss this ultrasound finding and your risk with your doctor.
Can Down syndrome and other trisomies be diagnosed on ultrasound?No. Ultrasound allows a doctor to visualize a fetus and to detect some physical markers that might indicate an increased risk, but ultrasound alone can never make a diagnosis of a chromosome abnormality such as Down syndrome or trisomy 18. The only way to make a diagnosis of Down syndrome or other chromosome abnormality in a pregnancy is an amniocentesis test.
It is important to remember that while an ultrasound cannot diagnose Down syndrome or other chromosomes abnormality, a normal ultrasound is reassuring but it is not a guarantee there the baby is perfectly healthy. There is not prenatal test that can rule out all birth defects or forms of mental retardation.
How Are Chromosome Abnormalities Diagnosed During a Pregnancy?
The only way to tell is a fetus has a chromosome abnormality such as Down syndrome or trisomy 18 is to perform an amniocentesis which is usually performed in the second trimester. During an amniocentesis, a thin needle is inserted through the woman’s abdominal wall into the uterus and a small amount of the fluid surrounding the baby is removed. The fetal chromosomes are studied using either a FISH test and/or a karyotype test. The results from FISH testing, while limited, are usually available in 3 to 4 days and the results of a full karyotype analysis usually take 2 weeks. The results of this testing are over 99% accurate in diagnosing Down syndrome and other trisomies.
Because choroid plexus cysts are not seen until the second trimester of pregnancy, CVS testing (done in the first trimester of pregnancy) is not an option. If you have had a normal CVS test, then there is no reason to be concerned about the finding of choroid plexus cysts on your ultrasound. The choroid plexus cyst is then considered a normal human variation which is not known to be harmful to the baby.
If you decide to have an amniocentesis and the results are normal, no further testing is necessary. The choroid plexus cyst is then considered a normal human variation which is not known to be harmful to the baby.
The Bottom Line
Choroid plexus cysts, while sounding scary, are a common ultrasound finding. Most often they are a normal findings that has no significance for the pregnancy. Rarely (less than 1% of the time), they can be a marker that the fetus has an underlying chromosome abnormality such as Down syndrome or trisomy 18. If chroid plexus cysts are seen on your ultrasound, you should discuss this finding with your doctor or a genetic counselor to see what your risk is and to discuss further testing options if appropriate.
Newberger, D., Down Syndrome: Prenatal Risk Assessment and Diagnosis. American Family Physician. 2001.
ACOG Committee on Practice Bulletins. ACOG Practice Bulletin No. 77: Screening for fetal chromosomal abnormalities. Obstet Gynecol. 2007 Jan;109(1):217-27.