I. What is a choroid plexus cyst? Choroid plexus cysts are caused by cerebrospinal fluid filling the neuroepithelial folds. The typical ultrasound imaging presentation is a small (usually less than 1 cm), well-defined, anechoic structure located within the choroid plexus. Multiple presentations may occur, ranging from unilateral solitary cysts to bilaterally separated multiple cysts. The fetal choroid plexus begins to develop at 6-7 weeks of gestation and grows rapidly, covering 75% of the lateral ventricular cavity by 9 weeks of gestation. It develops to an adult appearance by 20 weeks of gestation. In the absence of other CNS or extra-CNS abnormalities and in the absence of chromosomal aneuploidy risk factors, a simple choroid plexus cyst is considered a normal variant, regardless of its shape, size, or whether it is biased to one side or the other. Its usually disappears by late gestation, and even if it persists it is mostly asymptomatic and is a benign lesion. What should I do if I find a choroid plexus cyst? Fetuses with choroid plexus cysts combined with other malformations are at increased risk for chromosomal abnormalities, especially trisomy 18. Therefore, fetuses with detected choroid plexus cysts need to undergo a comprehensive ultrasound examination of the anatomical structures to guide further counseling and evaluation, especially fetal karyotyping. The significance of the presence of choroid plexus cysts 1. 30%-50% of fetuses with trisomy 18 will develop choroid plexus cysts, compared to only 1%-3% of all mid-term pregnancies. 2, Most large studies have shown that if a choroid plexus cyst is present but detailed structural examination confirms that the fetus is otherwise completely normal (including examination of the face, heart, great vessels and extremities), it is fairly safe to assume that the fetus has a normal karyotype. 3. If the fetus is able to loosen and remain unfolded, it is unlikely to be trisomy 18. 4, If a fetus with otherwise normal midterm pregnancy is found to have a choroid plexus cyst, one can be fairly confident that the free DNA screen is normal and that invasive testing (amniocentesis) is not required. 5. Prenatal detection of simple choroid plexus cysts is not associated with poor long-term developmental outcome.