Cervical fold thickness is the thickness of the skin at the back of the fetal neck measured in midtrimester. The cervical fold thickness is now considered to be clinically significant at 16 to 18 weeks of gestation ≥5 mm or at 19 to 24 weeks of gestation ≥6 mm. Factors such as gestational week, fetal position and the presence or absence of umbilical cord encirclement can affect the measurement of cervical fold thickness. It has been reported that 80% of neonates with trisomy 21 have thickened cervical folds, and the same finding has been found in neonates with trisomy 13, trisomy 18, and 45, X. Subsequent studies have consistently demonstrated that cervical fold thickening is the most sensitive and important ultrasound genetic marker for screening for trisomy 21 in mid-pregnancy. It is generally accepted that a cervical fold thickness of ≥6 mm increases the fetal risk of trisomy 21 by 17-fold. Recommendations for clinical management: If a thickened cervical fold is detected at mid-pregnancy screening, karyotyping should be recommended. Some studies have suggested that thickened cervical folds are also associated with congenital heart disease, so ultrasound of the fetal heart has also been suggested. However, there are different views that cervical fold thickening is different from the thickening of the nuchal translucency found in early pregnancy and that detailed fetal cardiac ultrasound is not needed if the chromosomal examination is normal.