NT scan should be done between 11 to 14 weeks and to get correct results of double marker test, you need to do the test as soon as possible after doing NT scan. If you have not done the NT scan and double marker tedt as i have mentioned, the results can be unreliable. But if you have done NT scan and double marker test at correct timing and still the resk for Downs syndrm is high, then you should undergo amniocentesis.