Are choroid plexus cysts an indication for second-trimester amniocentesis?

Previous series that described fetuses with choroid plexus cysts have been too small to determine whether there is an association with trisomy 18 sufficiently high to warrant amniocentesis. To address this issue, we studied the incidence of choroid plexus cysts and other ultrasonographic abnormalities in 26 consecutive fetuses (13.5 to 36 weeks' gestation) with trisomy 18. Twenty of these 26 fetuses had major sonographic anomalies suggestive of aneuploidy. Seventeen of these 26 fetuses were 15 to 20 weeks and 5 of 17 (30%) had choroid plexus cysts. Six of our total 26 affected fetuses had no sonographic anomalies and therefore, on the basis of our data, 30% of these (1.8 fetuses) with trisomy 18 would have choroid plexus cysts without other findings. The incidence of choroid plexus cysts in all second-trimester fetuses (including normal fetuses and those with trisomy 18) is reportedly 1%. Given the known incidence of trisomy 18 ( 3 10,000), we calculated a total presumptive sample of 86,667 patients to yield our 26 fetuses with trisomy 18. Our hypothetical sample has 86,641 (86,667 - 26) fetuses without trisomy 18, 858 of which would have choroid plexus cysts. Thus there would be one fetus with trisomy 18 for every 477 normal fetuses with choroid plexus cysts with no other defect seen. If amniocentesis were done to seek trisomy 18 in all second-trimester fetuses with choroid plexus cysts, two normal fetuses would be lost for every one with trisomy 18 identified.