My little pregnancy book says NT scans estimate the risk of Downs based on the measurement of the thickness of the fluid-filled space at the back of the neck, as well as the mothers age and the stage of pregnancy. Detects 60-70 per cent of bubs with Downs and may indicate other fetal abnormalities.
Its an ultrasound done at 12-13 weeks.

I wouldnt be to concerned. U/S's are normally done at the 12 week mark anyway so go and enjoy it.