Nuchal Thickness at 20 weeks. It true?

It is not absolutely true. For the screening of Downs syndrome in 2nd trimester (12-28wks), we do a USG and look for some soft markers (called so because these can be present in normal and disappears with increase in duration of pregnancy). The criteria is that if 2 or more soft markers are present, it increases the probability of having Downs syndrome. And the nuchal fold thickness >= 6 mm is just one of them, so if it's the only finding, nothing to worry about. The soft markers are: 1. Nuchal fold thickness >=6 mm 2. Hypoplastic nasal bone 3. Short femur 4. Short humerus 5. Short ear length 6. Short frontal lobe of brain 7. Intra cardiac echogenic foci 8. Echogenic foci in bowel 9. Sandal gap 10. Mild pyelectasis 11. Choroid plexus cyst. Also to supplement, we have some serum tests also in both 1st (0-12wks) and 2nd trimester of pregnancy.