From wikipedia-
During embryological
development, some fetuses may form choroid plexus cysts. These fluid-filled cysts can be detected
by a level II ultrasound (18–20 weeks gestation). The finding is relatively common, with a prevalence of ~1%. Choroid plexus cysts (CPC) can be an isolated finding, which confers a 1% (variable based on population studied) risk of fetal aneuploidy. The risk of aneuploidy increases to 10.5-12% if other risk factors or ultrasound findings are noted.