Marker Screening for Trisomy 18

The choroid plexus, a structure within the brain ventricles that produces cerebrospinal fluid, is usually easy to see by sonography between week 12 and 22.

Early in pregnancy, the choroid plexus has a butterfly shaped appearance and seems to almost fill the brain’s ventricles. As the baby grows, the choroid plexus is visible as a tear drop shaped gray appearing structure within the right and left ventricles of the baby’s brain. In some pregnancies a small clear space or “cyst” is seen within the choroid plexus. Choroid plexus cysts can be seen within the choroids plexus of the right ventricle, of the left ventricle or in both ventricles. They can range in size from very tiny up to nearly ½ inch in diameter.

Babies with choroid plexus cysts seen in utero are at increased risk of having chromosome problems, particularly trisomy 18, also known as Edwards syndrome. If choroid plexus cysts are seen on ultrasound, the next step is to perform a comprehensive ultrasound. These sonograms performed at our centers are done by experienced perinatal sonographers and obstetricians with subspecialty training in maternal-fetal medicine.

When a comprehensive sonogram is normal except for the choroid plexus cysts, there is controversy regarding how the information should be used for counseling. Genetic counseling is recommended to discuss this finding, other screening results, and options such as amniocentesis. Isolated choroid plexus cysts are seen in 1 percent of pregnancies; they carry an increased risk for trisomy 18. If the chromosomes are normal, choroid plexus cysts are considered a variant of normal anatomy and have no known implications for central nervous system development or neurologic outcome of the newborn.

For more information on choroid plexus cysts, click here.

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