Choroid plexus (CP) cysts are commonly detected on routine mid-trimest
er ultrasound scan. When associated anomalies are detected, the risk i
s sufficient to justify an invasive diagnostic test such as amniocente
sis. However, the risk when no associated anomalies are detected is mu
ch less well defined. This information is required to determine the ap
propriate management in cases of apparently isolated CP cysts. We thou
ght the only way to resolve the difficulties in counselling prospectiv
e parents was to conduct a prospective study in a large unselected pop
ulation. A registry of fetal CP cysts detected over 3 years in the Yor
kshire Region was compiled and we identified 524 CP cysts. These cases
were then amalgamated and analysed with 1361 cases from prospective s
tudies reported in the world English literature and a further 71 unpub
lished cases identified from a 2 year prospective series from Ninewell
s Hospital, Dundee. The risk of chromosomal abnormalities was 1 in 150
(95% CI 1 in 85, 1 in 261) when no fetal anatomic abnormalities, apar
t from the CP cysts themselves, were detected antenatally. The risk in
creased to approximately 1 in 3 if any other associated ultrasound abn
ormalities were detected antenatally. The risk did not appear to be re
lated to whether or not cyst size diminished as gestation progresses,
whether they were unilateral or bilateral, and whether they were small
or large in size (60-80% <10 mm). 76% of aneuploidic cases were triso
my 18 and 17% were trisomy 21. The risk of Down's syndrome in fetuses
with CP cysts but no other anomalies detected antenatally is 1 in 880.
The probability of a chromosomal abnormality is high when CP cysts ar
e associated with any other antenatally detected anomaly, indicating a
clear need to offering amniocentesis. The predictive value is much lo
wer when no other anomalies are detected. In such cases, it is probabl
y advisable to regard CP cysts as an indication for detailed ultrasoun
d assessment, rather than invasive testing.