i think around a 2.7 is 1 in 200 chance of downs (going by nuchal measurment alone) so obviously if the fold is thicker the risk is higher.
blood tests are also done at around 16 weeks which give an idea of risk or you can opt for a combined blood and nuchal (privately) to get a much more accurate risk.
If you are thought to be at greater risk than 1 in 250 then you will be told of other tests that will give a definate result.
Because of my age my risk was 1 in 60'ish so i opted for an amnio. Not necessarily the easy option as to have one will leave you open to miscarriage for the rest of your pregnancy. Your midwife/doctor should be giving you all this info but I would advise that you do your own research too as I was not offered an amnio and had to ask for it.