Choroid Plexus Cysts are a common ultrasound finding during the 2nd
trimester of pregnancy. To identify a choroid plexus cyst, it must be
imaged in two orthogonal planes and be greater than or equal to 3 mm in
size. In most instances, these are a normal variant. However, because many
fetuses with chromosomal aneuploidy (primarily Trisomy 18, but also Trisomy
21) have been found to have choroid plexus cysts, this ultrasound finding
indicates an increase in the risk that one of these chromosome
abnormalities is present in the fetus. There is controversy regarding the
magnitude of this risk. We do not feel that the risk is dependent on the
number and laterality of the cysts. We estimate that the risk of Trisomy 18
or Trisomy 21 is approximately 1% when isolated choroid plexus cysts (of
any number) are present. This risk is modified by factors such as maternal
age, maternal serum screen results, and ultrasound findings. At present,
however, there is no agreed-upon algorithm for combining these separate
factors to determine an overall risk. Since abnormalities of the hands,
feet, and heart are common in trisomy 18, we specifically mention in the
ultrasound report that they have been evaluated.
We recommend that genetic counseling be provided for this finding. If,
following genetic counseling, the patient chooses not to have amniocentesis
and the initial ultrasound was performed prior to 17 weeks gestation, or if
the anatomy was not well-visualized, we recommend a follow up ultrasound at
20-22 weeks gestation for evaluation of fetal anatomy and growth. The
choroid plexus cysts usually resolve by the 3rd trimester of pregnancy
(whether or not the fetus has a chromosome abnormality) and are not
clinically significant. The follow up ultrasound is recommended solely as a
means of better visualizing the anatomy and evaluation of possible IUGR to
decrease the chance of aneuploidy.
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