 How to define a normal or abnormal nuchal translucency measurement can be difficult.The use of a single millimeter cutoff (such as 2.5 or 3.0 mm) is inappropriate because nuchal translucency measurements normally increases with gestational age (by approximately 15% to 20% per gestational week from 10 to 13 weeks).At 12 weeks of gestational age, an "average" nuchal thickness of 2.18mm has been observed; however, up to 13% of chromosomally normal fetuses present with a nuchal translucency of greater than 2.5mm.