I'm 20th Week Pregnant Woman. And My Age Is 28years.it

I'm 20th week pregnant woman. And my age is 28years.it is my first pregnancy. One of Dr suggested for NT scan. But we were delayed to get scan for personal reasons. After a week of suggested time we went to radiologist, but he said its too late to get the approximate result. And my family Dr. Said its not that much necessary if we are not having any familial chromosomal history and downs syndrome. Still we are having some hesitation about my fetal health. Please clarify my doubt about NT scan. And Is it necessary for everyone. If it failed to get what will be the next scan at this point of time. Now we are going for tiffa scan