<prompt> Chorionic villous sampling is useful in all the following exceptTay-Sachs diseaseSpina bifidaThalassanemiaDown syndrome  <response>: Chorionic villous sampling CVS is generally performed at 10 to 13 weeks. 1. Second-trimester amniocentesis - for genetic diagnosis is usually performed between 15 and 20 weeks. 2. Early amniocentesis - this is performed between 11 and 14 weeks CVS for trophoblastic cells derived embryologically from the same feilized eggs as the fetus is usually done during gestational weeks 11-13 The advantage of CVS is that the results are available early in pregnancy 