<prompt> Chorionic villous sampling is needed in all exceptNeural tube defectsDowns syndromeThalassemianoonan syndrome  <response>: Chorionic villous samplingCVS is done at the 10 - 13 week of gestation to diagnose conditions which could be debilitating or life threatening for the unborn fetus. Neural tube defects are best assessed at a later gestation around 20 weeks though the earliest to be picked up is ANENCEPHALY which can be picked up by 10-12 weeks with USG. The most specific indicator for Neural tube defects is the acetylcholinesterase 