To analyze the prenatal diagnostic value of ultrasound for umbilical cord knot (UCK) and the relationship between umbilical cord knot and perinatal outcome.

A retrospective analysis was conducted on the prenatal ultrasound and clinical data of newborns or induced fetuses in late pregnancy at the First Affiliated Hospital of Anhui Medical University from January 2015 to December 2019. The incidence of umbilical cord knot, its relationship with fetal perinatal outcome, and the diagnostic value of prenatal two-dimensional and three-dimensional ultrasound in umbilical cord knot and its image features were summarized and analyzed. The fetuses who were diagnosed as having umbilical cord knot after delivery were included as an umbilical cord knot group, and 200 normal fetuses were randomly selected as a non-umbilical cord knot group. The differences of ultrasonic parameters of the middle cerebral artery and the umbilical artery were compared between the two groups by independent sample t-test (normal distribution) or rank sum test (skewed distribution).

During 5 years, there were 40 346 fetuses born at our hospital in the third trimester, including 209 cases of stillbirths (23 died of umbilical cord complications). Ninety-nine (0.25%, 99/40 346) fetuses were confirmed with umbilical cord knot after delivery, of whom three (3.03%, 3/99) died of umbilical cord knot, accounting for 1.44% (3/209) of fetal deaths during late pregnancy and 13.04% (3/23) of umbilical cord complications. Prenatal 2D ultrasound showed 15 cases of suspicious umbilical cord knot, of which 9 were diagnosed by 3D ultrasound and were confirmed after delivery. It was difficult to display the characteristic images of umbilical cord knot by 2D ultrasound. 2D images can be classified as excessively distorted, stacked, or cloverleaf. 3D ultrasound can clearly identify the true and false umbilical cord knot, which can be divided into loose umbilical cord knot, tight umbilical cord knot, spiraling umbilical cord, and twisted and stacked umbilical cord. The resistance index of the middle cerebral artery in the umbilical cord knot group (0.75±0.07) was significantly lower than that in the no-umbilical cord knot group (0.76±0.06; t=1.988, P=0.048).

2D ultrasound combined with 3D ultrasound can improve the prenatal diagnostic accuracy for umbilical cord knot. The prenatal diagnosis of umbilical cord knot is beneficial for obstetricians to choose the proper time and way of delivery to terminate pregnancy to avoid perinatal adverse outcomes. The presence of umbilical cord knot cannot be ruled out when the ultrasonographic blood flow parameters of the middle cerebral artery and umbilical artery are normal.