The best way of diagnosing Trisomy-21 during second trimester of pregnancy is:

A triple marker test, done to detect the chances of downs syndrome is a screening test whereas a amniocentesis is a definitive test done for confirmation of downs. detection of downs syndrome: Serum Markers in the first ( Like PAPP-A 7 bHCG) and second trimester( Triple marker with bHCG, AFP, uE3 or Quadruple with Inhibin added) USG in First trimester (like Nuchal translucemcy and Nasal bone detection) & second trimester ( like anomaly scan) Definitive tests in the first trimester like Chorionic villus sampling, second trimester like Amniocentesis cell free DNA detection of the fetus can be done in the maternal circulation & this can inform the Karyotypic and genetic anomalies with 98% sensitivity.