The best way of diagnosing Trisomy 21 during second trimester of pregnancy is:
Triple marker estimation
Nuchal skin fold thickness measurement
Chorionic villus sampling
Amniocentesis
The main use is in diagnosing chromosomal disorders like trisomy 21.Amniotic fluid cells are cultured and karyotyping done,which may take up to 3 weeks (refer pgno:502 sheila textbook of obstetrics 2 nd edition)
Diagnosis in obstetrics