However, large cysts can cause hydrocephalus. Genetic causes
There is a possible association between ultrasound-detected fetal CPCs and Trisomy 18. It is not correlated to the presence of Trisomy 21 (Down syndrome). Therefore, genetic counseling is often recommended to provide more information about fetal CPCs, to answer questions and concerns, and to outline available options such as amniocentesis or a blood test from the mother.Generally the risks are very low if there are no other risk factors. If no additional abnormalities are detected by a thorough "level II" ultrasound, the likelihood the fetus has trisomy 18 is very low.A meta-analysis of 8 studies between 1990 and 2000 with choroid plexus cysts that were identified in second-trimester (an incidence of 1.2%). The incidence of the cysts in women younger than 35 was 1% (n=1017). The study found no cases of trisomy 18 in fetuses with cysts whose mother was younger than 35. The study concluded that "there is no evidence that detection of isolated choroid plexus cyst in women who are <35 years of age increases the risk of trisomy 18".Other factors which may have a bearing on the babys chances of developing chromosome problems include:
mothers age at the expected date of delivery
the results of serum screening; XAFP triple testing or quad screening
evidence of other "fetal findings" seen at the time of the ultrasound that may suggest a chromosome problem

References
Further reading
Lun, Melody P.; Monuki, Edwin S.; Lehtinen, Maria K. (2015). "Development and functions of the choroid plexus–cerebrospinal fluid system". Nature Reviews Neuroscience. 16 (8): 445–457. doi:10.1038/nrn3921. PMC 4629451.
PMID 26174708. Rochon, Meredith; Eddleman, Keith (March 2004). "Controversial ultrasound findings". Obstetrics and Gynecology Clinics of North America. 31 (1): 61–99. doi:10.1016/S0889-8545(03)00123-2. PMID 15062448. McKinney, Alexander M. (2017). "Choroid Plexus: Normal Locations and Appearances". Atlas of Normal Imaging Variations of the Brain, Skull, and Craniocervical Vasculature. Springer International Publishing. pp. 177–237. doi:10.1007/978-3-319-39790-0_11. ISBN 978-3-319-39790-0. Aktürk, Erhan; Güler, Aşkın Evren (26 March 2020). "Review of fetal choroid plexus cysts: A cross-sectional study on 9244 pregnant women". Journal of Surgery and Medicine. doi:10.28982/josam.706887. DeRoo, TR; Harris, RD; Sargent, SK; Denholm, TA; Crow, HC (December 1988). "Fetal choroid plexus cysts: prevalence, clinical significance, and sonographic appearance". American Journal of Roentgenology. 151 (6): 1179–1181. doi:10.2214/ajr.151.6.1179. PMID 3055893. Chitkara, Usha; Cogswell, Carolyn; Norton, Karen; Wilkins, Isabelle A.; Mehalek, Karen; Berkowitz, Richard L. (August 1988). "Choroid Plexus Cysts in the Fetus: A Benign Anatomic Variant or Pathologic Entity? Report of 41 Cases and Review of the Literature". Obstetrics & Gynecology. 72 (2): 185–189. PMID 3292976.