Nuchal thickness is very specific to gestational age. It has to be taken when the CRL I.e baby s length is 45 to 84mm. If it is taken beyond that it cannot be considered. You need to assess the combined risk of NT and double marker to assess the risk of chromosomal abnormality and then decide if invasive testing is required for confirmation. Again double marker test is done between 11to 14 week s. If you have missed it do a triple marker at 15-16 weeks and then decide.