A thickening of the skin thickness in the posterior aspect of the fetal neck. A nuchal fold (NF) measurement is obtained in a transverse section of the fetal head at the level of the cavum septum pellucidum and thalami, angled posteriorly to include the cerebellum. The measurement is taken from the outer edge of the occiput bone to the outer skin limit directly in the midline. An NF measurement greater than 5 mm at 14 to 17+6 weeks of gestation, or 6 mm at 18 to 28 weeks has been associated with a markedly increased risk for Down syndrome. The phenotype is Human Phenotype Ontology entry HP_0000474 (Thickened nuchal skin fold). Also known as: Excess nuchal skin, Increased nuchal fold, Increased nuchal fold thickness, Thick nuchal fold, Thickened nuchal skin, Thickened skin folds of neck, Thickened skin over the neck.