<prompt> Most sensitive sonological indicator for aneuploidy isGestational sac volumeCrown-rump lengthNuchal transluscencySerum P HCG level  <response>: Ans. is c i.e. Nuchal translucency Rumack writes - The most effective sonographic marker of trisomy 21 and other chromosomal defects is increased nuchal translucency NT thickness at 11 to 14 weeks gestation. 