There are many strategies available to screen for
chromosomal abnormalities, including combined test,
triple test, quad test, integrated screen, stepwise sequential
screen and contingent sequential screen [3]. All of
these approaches provide an adjusted risk for Down
syndrome and trisomy 18, but they do not exclude the
possibility of an affected fetus, because the test sensitivity
is less than 100%. Therefore, amniocentesis is still
the only diagnostic test in current use that is valid for
diagnosis.