Targeted imaging for foetal anomalies or tiffa scan is nowadays advised between 17-20 weeks of pregnancy
It helps to analyse foetal anomalies at an early stage so that if the foetus is detected with anomalies that are not compatible with life,abortion
is carried out and the pregnancy period is not useleasly prolonged.
Tripple marker test can onlu detect down's syndrome whereas tiffa scan xan detect many other anomalies too though not all.
Tiffa scan is a simple ultrasound
and you need not worry about it.