3mm in 1st trimester identifies 67% of fetus with trisomy 21. Increased nuchal fold thickness (> 6mm) is the most sensitive and specific single ultrasound marker for the mid trimester detection of down syndrome. Extra Edge : Nuchal translucency is the most impoant sonographic marker of aneuploidy in the first trimester whereas, Nuchal fold thickness (NFT) is the most impoant sonographic marker of aneuploidy in second trimester. NT is measured from external surface of the skull to the internal surface of the skin. NFT is measured from the external surface of occipital bone to the external surface of skin. NFT is useful for screening between 14-23 weeks. Cut off value for NFT is 5mm (above which it is considered abnormal). Note: In the first trimester 2 most reliable ultrasonographic markers for detecting Down syndrome? Increased muscal translucency Absent nasal bone.">