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The umbilical cord is the vital connection between the fetus and the placenta. Umbilical cord development begins during the embryonic period, around week 3, with the formation of the connecting stalk. By week 7, the umbilical cord has fully formed, composed of the connecting stalk, vitelline duct, and umbilical vessels surrounding the amniotic membrane. The umbilical vessels carry the fetal blood back and forth to the placenta, with the umbilical vein carrying oxygenated blood with nutrients from the placenta to the fetus and the umbilical arteries transporting deoxygenated blood with waste products from the fetus to the placenta. Embryonic structures regress near the end of the first trimester, leaving the umbilical cord composed of 2 umbilical arteries and 1 umbilical vein surrounded by a gelatin-like extracellular matrix known as Wharton jelly. Elongation of the umbilical cord occurs primarily in the second trimester. The average umbilical cord is 50 to 60 centimeters in length, 2 centimeters in diameter, with up to 40 helical turns. Abnormalities of the umbilical cord can lead to increased morbidity and mortality of the fetus.

A single umbilical artery occurs in less than 1% of all pregnancies due to primary agenesis or secondary atrophy. Over half of these are isolated single umbilical arteries, but the anomaly is also associated with an increased risk of congenital and chromosomal abnormalities. Additionally, a single umbilical artery correlates with prematurity and intrauterine growth restriction.[1][4][26][27][28] When the umbilical cord inserts near the margin of the placenta instead of the center, it is referred to as marginal cord insertion or battledore placenta; this occurs at a rate of 9% in singleton pregnancies, with an increased rate in twin pregnancies (24 to 33%). Marginal cord insertion is associated with intrauterine growth restriction, preterm labor, and fetal distress.[3][4][28] Velamentous cord insertion is a type of abnormal insertion occurring in 1 to 2% of pregnancies in which the umbilical vessels begin to spread out before reaching their normal insertion site at the center of the placenta. In this anomaly, the vessels travel separately between the amniotic and chorionic membranes before reaching the placenta. This area lacks the normal protection of Wharton jelly, leaving it susceptible to compression and rupture. Velamentous cord insertion increases the risk of adverse outcomes in the perinatal period due to vasa previa and placental abruption.[3][4][28][29] Vasa previa occurs in about 0.04% of pregnancies when fetal vessels are located between the cervix and the fetal presenting part and can result from velamentous cord insertion or vessels traveling between lobes of the placenta. If pregnancy progresses to rupture of membranes, vasa previa presents with the combination of painless vaginal bleeding and fetal heart tones showing signs of distress.[4][28][29][30] Loss of Wharton jelly occurs most commonly near the fetal insertion site, but also presents near the placental insertion site. The loss of this protective material leaves the umbilical cord vessels susceptible to compression due to twisting and knotting. The absence of Wharton jelly at any location on the umbilical cord increases the risk of intrauterine fetal demise as well as adverse perinatal outcomes due to compression of vessels during labor. Loss of Wharton jelly may be diagnosable before delivery, with a decreased diameter of the umbilical cord visualized by ultrasound.[4][22]

Loss of Wharton jelly occurs most commonly near the fetal insertion site, but also presents near the placental insertion site. The loss of this protective material leaves the umbilical cord vessels susceptible to compression due to twisting and knotting. The absence of Wharton jelly at any location on the umbilical cord increases the risk of intrauterine fetal demise as well as adverse perinatal outcomes due to compression of vessels during labor. Loss of Wharton jelly may be diagnosable before delivery, with a decreased diameter of the umbilical cord visualized by ultrasound.[4][22] When the vitelline duct fails to regress completely during the embryonic period, it can lead to an abnormal outpouching of the intestines, known as a Meckel diverticulum. This outpouching persists in about 2% of neonates, usually measures about 2 inches in length, and is typically located in the ileum, about 2 feet from the ileocecal valve (following the rule of 2s). Partial regression of the duct can lead to a vitelline fistula, a fibrous band-like connection between the umbilicus and intestine, or a vitelline cyst, which is an abnormal collection of fluid within the remaining duct. In most cases, these anomalies are asymptomatic, but the abnormal connection is associated with an increased risk of internal hernia, volvulus, and intussusception.[2][5] Funisitis is the migration of fetal neutrophils out of the bloodstream and into the umbilical cord. This migration process initiates with the release of neutrophil chemokines, such as interleukin-8 and granulocyte chemotactic protein. Funisitis is most commonly present in the setting of intraamniotic infection, specifically chorioamnionitis, and is part of the fetal inflammatory response syndrome, which indicates a high risk of preterm labor and increased neonatal morbidity. This process is identified microscopically after delivery, but because mature neutrophils are not present in fetal blood, it is not typically present before 20 weeks of gestation.[31]

Funisitis is the migration of fetal neutrophils out of the bloodstream and into the umbilical cord. This migration process initiates with the release of neutrophil chemokines, such as interleukin-8 and granulocyte chemotactic protein. Funisitis is most commonly present in the setting of intraamniotic infection, specifically chorioamnionitis, and is part of the fetal inflammatory response syndrome, which indicates a high risk of preterm labor and increased neonatal morbidity. This process is identified microscopically after delivery, but because mature neutrophils are not present in fetal blood, it is not typically present before 20 weeks of gestation.[31] If the umbilical cord becomes too long in utero, there is an increased risk that it can become wound around the fetus or even become tied into a knot due to fetal movement. If the cord becomes wound around the fetal neck, it is referred to as a nuchal cord. The incidence of nuchal cord is estimated at up to 29% at term, with the incidence increasing with advancing gestational age. When the fetus descends during labor, increased torsional forces on the umbilical cord can decrease blood flow through the umbilical vessels and lead to signs of fetal distress and acidosis. When discovered, expedient reduction of the nuchal cord is important to return proper blood flow to the fetus and avoid prolonged asphyxia.[4][17] Similarly, knots that form in utero are associated with longer umbilical cords. Loose umbilical knots present no danger to the fetus on their own, but when the knot tightens, the increased compression on the cord initially collapses the thin-walled vein before the thicker-walled arteries. This tightening of the knot can happen in utero and during labor, leading to signs of fetal distress, asphyxia, or even intrauterine fetal demise.[4][17][21]

Similarly, knots that form in utero are associated with longer umbilical cords. Loose umbilical knots present no danger to the fetus on their own, but when the knot tightens, the increased compression on the cord initially collapses the thin-walled vein before the thicker-walled arteries. This tightening of the knot can happen in utero and during labor, leading to signs of fetal distress, asphyxia, or even intrauterine fetal demise.[4][17][21] Additionally, longer umbilical cords are more likely to lie between the cervix and the presenting fetal part during delivery, leading to possible umbilical cord prolapse with rupture of membranes. Umbilical cord prolapse is diagnosed by palpation of the umbilical cord within the vagina along with changes in the fetal heart tracing indicating fetal distress, such as recurrent and prolonged decelerations. Management of this condition is the expedient delivery of the baby, mostly via cesarean section, but operational vaginal delivery may be performed if it is determined to be the faster route.[4][32] In monochorionic monoamniotic twin gestations, the fetuses share the same amniotic cavity within the uterus, leading to possible cord entanglement. Similar to a knotted cord, tightening of the cord entanglement and increasing entanglement can cause compression of fetal vessels, leading to intrauterine fetal demise. Cord entanglement can be detected in utero by ultrasound, but studies suggest that prenatal diagnosis has not shown improved neonatal outcomes.[4][28] Other rare abnormalities include a persistent right umbilical vein, an umbilical artery aneurysm, an umbilical cord cyst, an umbilical hemangioma, and an umbilical teratoma.[4]