Nuchal edema is considered present in a fetus at gestational week 14 or more if in the midsagittal plane of the neck, there is subcutaneous edema that produces a characteristic tremor on ballotment of the fetal head. This constitutes the severe end of the spectrum of increased nuchal fold thickness, which is defined as soft-tissue thickening of 6 mm or more, seen in the suboccipitobregmatic view of the fetal head. Nuchal edema may be confined to the neck or it may be generalized, as part of hydrops fetalis. [from
HPO]