Obstetrics and Gynecology

Velamentous insertion

Velamentous insertion of the umbilical cord

1. What every clinician should know

Clinical features and incidence

Velamentous insertion of the umbilical cord refers to insertion of the cord into the membranes rather than directly into the placenta. Fetal vessels then traverse the membranes freely, unprotected by Wharton's Jelly or placental tissue, to insert into the placenta (Figure 1). When velamentous vessels overlie the cervix, the condition is known as vasa previa. In vasa previa, spontaneous or artificial rupture of the membranes carries an extremely high risk of fetal exsanguination and death.

Figure 1.

Velamentous cord insertion. The cord is seen at the upper part of the picture, inserting into the membranes (arrow). Free vessels then run through the membranes to insert into the placenta (p).

When a velamentous insertion occurs away from the cervix (i.e. when there is no vasa previa), most pregnancies proceed normally with no complications. Occasionally, however, intrauterine growth restriction may occur. Also, rarely, velamentous vessels that are not in the lower segment of the uterus may rupture spontaneously, resulting in fetal death. However, this is exceedingly uncommon.

When velamentous cord insertion is present in a monochorionic twin pregnancy, there may be unequal placental sharing with risks of growth restriction and death in the twin whose cord inserts into the membranes. Also, velamentous cord insertion in a monochorionic twin pregnancy increases the risk for Twin-Twin transfusion syndrome. Velamentous cord insertion also may exist in association with chromosomal abnormalities and syndromes, and in association with fetal anomalies.

Velamentous insertion occurs in approximately 1% of all pregnancies. Velamentous cord insertion is more common among multi-fetal pregnancies, and has been estimated to occur in up to 10% of twin pregnancies, with increasing incidence with increasing number of fetuses in a multifetal gestation.

2. Diagnosis and differential diagnosis

Establishing the diagnosis

The gold standard for diagnosis of velamentous insertion of the cord is based on inspection of the placenta and cord following delivery. When the cord insertion is specifically looked for using prenatal ultrasound, velamentous insertion may be diagnosed prenatally. In the absence of prenatal ultrasound, it is not possible to make the diagnosis prior to placental delivery. Velamentous insertion also should be suspected when the cord easily avulses from the placenta in the third stage of labor with little traction.

The ultrasound appearance of a velamentous insertion is of a cord that does not insert centrally into the placenta. Generally, location of the cord insertion into the placental body is possible using prenatal ultrasound, especially if color or power Doppler are used. Failure to recognize this highly suggests a velamentous insertion. Using a combination of gray scale ultrasound and color, power and pulsed wave Doppler, one may determine that the cord inserts directly into the membranes.

The appearance on gray scale ultrasound is of echolucent linear structures running along the uterine wall and ending in the edge of the placenta. Color and power Doppler will reveal flow through these structures. Pulsed wave Doppler will demonstrate a fetal umbilical arterial or venous signal through these structures, confirming fetal vessels.

Differential diagnosis

The differential diagnosis includes marginal cord insertion. In this, the cord inserts into the margin of the placenta into placental tissue and not into the membranes. Marginal cord insertion is not associated with any increased risks. In addition, chorioamniotic separation or subchorionic hemorrhage may be mistaken for velamentous insertion. In these cases, Doppler will typically not reveal any flow through the echolucent structures. Even when flow does exist, it should not have an umbilical arterial or venous waveform on pulsed Doppler examination.

3. Management

Antepartum

In the absence of vasa previa, velamentous insertion carries minimal risk. Detailed sonography should be performed at about 20 weeks to rule out associated fetal anomalies. Because of the risk of associated growth restriction, serial growth ultrasounds should be performed every 4 weeks starting at about 28 weeks of gestation. When velamentous insertion exists in a monochorionic twin pregnancy, care should be taken to evaluate the fetuses at regular intervals for unequal placental sharing.

The patient should report immediately should she experience reduced fetal movement or vaginal bleeding. Whether patients with velamentous insertion should have weekly fetal surveillance (biophysical profile or non-stress tests) is questionable. In cases associated with fetal growth restriction, fetal testing should be employed. Otherwise, obstetrical care should be as routinely done except in cases in which the velamentous insertion overlies the cervix (i.e. vasa previa). In such cases, the patient should be admitted to hospital at about 32 weeks, with delivery by elective cesarean scheduled for 35 weeks, or immediately should the membranes rupture, bleeding occur or labor ensue.

Intrapartum

When velamentous insertion is known to be present prior to the onset of labor, close continuous electronic fetal heart rate monitoring should be performed in labor. In the presence of a Category 3 or persistent Category 2 fetal heart rate tracing, cesarean delivery should be considered.

Postpartum

If velamentous cord insertion is known to exist prior to delivery, care should be taken with delivery of the cord to minimize the risk of cord avulsion. Every attempt should be made to avoid excessive traction prior to placental separation. However, if the cord is avulsed during the third stage of labor, manual removal of the placenta may be necessary.

4. Complications

The highest risk of velamentous insertion occurs with a vaa previa. In the absence of vasa previa, complications are rare. When velamentous insertion occurs with a monochorionic twin pregnancy, growth restriction or twin-twin transfusion syndrome may occur. Also, retained placenta may occur in patients with velamentous insertion. In these patients, caution should be taken to avoid excessive cord traction. When cord avulsion occurs, there may be a retained placenta with the risk of postpartum hemorrhage.

5. Prognosis and outcome

Fetal/neonatal outcomes

Velamentous insertion of the umbilical cord is generally associated with an excellent prognosis as long as there are no fetal vessels overlying the cervix. Fetal death or hemorrhage may occur with vasa previa.

Impact on mother's long-term health

B. Velamentous cord insertion in and of itself should have no impact on the long-term health of the mother

6. What is the evidence for specific management and treatment recommendations

Nomiyama, M, Toyota, Y, Kawano, H. "Antenatal diagnosis of velamentous umbilical cord insertion and vasa previa with color Doppler imaging". Ultrasound Obstet Gynecol. vol. 12. 1998. pp. 426-429.

(The authors attempted to determine whether umbilical cord insertion could be reliably and consistently located using ultrasound with color Doppler in 587 pregnancies at 18-20 weeks of gestation. They were successful in 586/587 (99.8%) of cases. The mean time taken was 20 seconds, and 95% of the time the cord insertion was found in less than 1 minute. This study demonstrates that screening for vasa previa can be done without excessive demands on manpower, equipment or time.)
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