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Velamentous umbilical cord insertion and vasa previa

Authors 
Charles J Lockwood, MD 
Karen Russo-Stieglitz, MD 

Section Editors 
Susan M Ramin, MD 
Deborah Levine, MD 

Deputy Editor 
Vanessa A Barss, MD 

Disclosures

All topics are updated as new evidence becomes available and our peer review process is complete.

Literature review current through: Mar 2013. | This topic last updated: Mar 20, 2012.

INTRODUCTION  — A velamentous umbilical cord is characterized by membranous umbilical vessels at the placental insertion site; the remainder of the cord is usually normal. Membranous vessels can also arise as aberrant branches of a marginally inserted umbilical cord or they can connect lobes of a bilobed placenta or the placenta and a succenturiate lobe. Because of the lack of protection from Wharton’s jelly, these vessels are prone to compression and rupture, especially when they are located in the membranes covering the cervical os (ie, vasa previa).

VELAMENTOUS UMBILICAL CORD

Definition  — In a velamentous umbilical cord insertion, the placental end of the cord consists of divergent umbilical vessels surrounded only by fetal membranes, with no Wharton's jelly. The length of the membranous vessels, ie, the distance between the end of the normal cord and the placental insertion, is highly variable.

Prevalence  — Velamentous insertion occurs in approximately 1 percent of singleton gestations [ 1 ], but is observed in as many as 15 percent of monochorionic twin gestations [ 2-4 ]. It is also more common in placenta previa than in normally located placentas. The prevalence may be slightly higher in stillbirths, particularly from multifetal pregnancies [ 4 ].

Pathogenesis  — The pathogenesis of velamentous cord insertion is unknown. The most popular hypothesis is that the cord is initially inserted centrally, but its location progressively becomes peripheral as one half of the placenta actively proliferates toward the well-vascularized uterine fundus (trophotropism) while the other pole involutes; the umbilical cord is unable to follow the migration of the placenta [ 5 ]. The association of velamentous cord insertion and placenta previa supports this hypothesis.

Clinical features

Ultrasound and gross examination  — On ultrasound and gross examination, the normal umbilical cord sheath is contiguous with the chorionic plate. With a velamentous insertion, the cord can end several centimeters from the placenta, at which point the umbilical vessels separate from each other and cross between the amnion and chorion before connecting to the subchorionic vessels of the placenta ( picture 1A-C ). This typically occurs at the margin of the placenta (within 1 cm of the placental edge), but can also occur at the apex of the gestational sac. In monochorionic twins, the velamentous vessels often occur in the dividing membranes.

On ultrasound examination, the umbilical vessels often lie parallel to the uterine wall as they enter the placental margin and connect to the subchorionic vasculature ( image 1 ). They remain immobile when the uterus is shaken by the ultrasonographer; in contrast, a loop of umbilical cord will move when the uterus is shaken [ 6 ]. Color Doppler imaging enhances identification of the vessels ( image 2 ).

Velamentous cords contain a single umbilical artery in about 12 percent of cases [ 5 ].

Clinical course  — Because the vessels are attached to the chorion, rupture of the fetal membranes may rupture the vessels, which can result in fetal exsanguination and death within minutes. This typically occurs when the membranous vessels are close to or cover the cervix; rarely, membranous vessels have ruptured in the absence of documented membrane rupture [ 7 ]. (See 'Vasa previa' below.)

The membranous vessels are also at risk of kinking and compression. Membranous vessels with longer lengths are more prone to kinking, while descent of the fetal presenting part increases the risk of compression when the membranous vessels are close to or cover the cervix. The subsequent reduction in blood flow can result in fetal heart rate abnormalities, and, if the reduction in blood flow is persistent or severe, fetal death may occur. Kinking and compression can also induce thrombosis of the vessels, which has been associated with placental infarction, amputation of fetal limbs or digits, and neonatal purpura [ 8,9 ].

In small series and case reports, velamentous insertion has been described in association with several obstetrical complications, including fetal growth restriction, preterm delivery, congenital anomalies, fetal distress, low Apgar scores, fetal death, and retained placenta [ 3,5,10-16 ]. However, large series (1000 cases) of marginal or velamentous insertion have not found a statistically significant increase in risk of any adverse outcome, suggesting there is no more than a slight increase in relative risk of adverse outcome attributable to velamentous insertion alone [ 17,18 ]. Series reporting a significantly higher risk of complications with velamentous cord insertion are often biased by more careful evaluation of the cord and placenta after a complicated delivery than after a normal delivery.

On the other hand, when only monochorionic twin pregnancies are examined, velamentous cord insertion site has been associated with discordant growth and intrauterine growth restriction [ 3,19 ].

Diagnosis  — The prenatal diagnosis of velamentous insertion is based upon the presence of characteristic sonographic findings (membranous umbilical vessels) at the placental cord insertion site. When color Doppler is used to enhance identification of the vessels, diagnostic sensitivities of 69 to 100 percent and specificities of 95 to 100 percent have been reported [ 1,20 ].  

A definitive diagnosis is made by gross examination of the placenta, cord, and membranes after delivery ( picture 1A-C ). (See 'Ultrasound and gross examination' above.)

Screening  — The American College of Radiology (ACR), the American Institute of Ultrasound in Medicine (AIUM), and the American College of Obstetricians and Gynecologists (ACOG) guideline for performance of obstetric ultrasound does not specifically recommend evaluation of the placental insertion site [ 21 ]. The guideline recommends that “the umbilical cord should be imaged, and the number of vessels in the cord should be evaluated when possible” with evaluation of the “umbilical cord insertion site into the fetal abdomen.”

Most experts do not recommend routine screening for velamentous insertion, as it is costly in low risk patients with no proven benefit while increasing anxiety and possibly unnecessary antepartum testing and intervention. However, some authors have opined that every second trimester ultrasound examination or every second trimester ultrasound in monochorionic twin pregnancies should evaluate the placental cord insertion. Even when this is done, the diagnosis of velamentous insertion may not be made in the prenatal period, and failure to make this diagnosis is not a breach of the standard of care [ 10 ].

Management  — There are no data from large or controlled studies on which to base management recommendations. There is no evidence that interventions such as late preterm induction of labor or scheduled cesarean delivery improve outcome. In our opinion, these pregnancies can be allowed to labor spontaneously and deliver vaginally, in the absence of additional pregnancy complications necessitating a different approach. However, we do not allow these pregnancies to continue beyond 40 weeks of gestation since decreasing amniotic fluid volume postterm may place the membranous vessels at increased risk of compression.

Given the risks described above, if ultrasound examination suggests the presence of a velamentous umbilical cord, we suggest the following:

• Detailed fetal anatomic survey, including evaluation for coexistent vasa previa
• Serial assessment of fetal growth, every four to six weeks
• Fetal heart rate tracings weekly, beginning at 36 weeks of gestation, to look for recurrent variable decelerations from kinking or compression
• Counseling patients to call their providers as soon as labor begins
• Delivery by 40 weeks of gestation

We monitor the fetal heart rate continuously intrapartum to identify signs of severe cord compression or vessel rupture. After delivery of the infant, no or very gentle traction should be placed on the umbilical cord to avoid avulsion, which could result in a retained placenta.

VASA PREVIA

Definition  — In vasa previa, fetal blood vessels are present in the membranes covering the internal cervical os. The membranous vessels may be associated with a velamentous umbilical cord (type 1 vasa previa) or they may connect the lobes of a bilobed placenta or the placenta and a succenturiate lobe (type 2 vasa previa) [ 22 ].

Prevalence  — The prevalence of vasa previa is approximately 1 in 2500 deliveries [ 23 ], but is much higher in pregnancies conceived following use of assisted reproductive technologies (prevalence as high as 1 in 202) [ 24-27 ]. The prevalence is also increased in second-trimester low-lying placentas or placenta previa (even if resolved), bilobed or succenturiate lobe placentas in the lower uterine segment, and multiple gestations [ 26,28 ].

Pathogenesis  — Pathogenesis is unknown, but is likely similar to that for velamentous cord insertion. (See 'Pathogenesis' above.)

Clinical features

Imaging  — On ultrasound examination, a linear sonolucent area consistent with a blood vessel passes over the internal os. Color Doppler flow mapping shows flow in the vessel ( image 3 ) with umbilical artery or vein waveforms. In about 90 percent of cases, the placenta is a previa, low-lying, bilobed, or succenturiate [ 29 ]. If the placenta is a previa or low lying, the umbilical cord insertion site is velamentous.

Physical examination  — Rarely, pulsating vessels in the membranes overlying the cervical os are palpable on digital examination.

Clinical course  — Suspected second trimester vasa previa may resolve over time; in one series, 3 of 18 cases resolved by the late third trimester and had an uncomplicated vaginal delivery [ 30 ].

Transvaginal ultrasound is sensitive and highly specific for the detection and resolution of vasa previa [ 22,30 ]. We suggest using the abdominal hand to push the presenting part cephalad in an attempt to better visualize the area of the internal os when the presenting part is engaged. This will help distinguish between a vasa previa, if persistent, and a funic presentation.

More commonly, the vasa previa persists and is at risk for rupture upon spontaneous or artificial rupture of the membranes; rarely, fetal bleeding occurs without membrane rupture. In most cases, bleeding rapidly results in fetal anemia and hypotension, leading to fetal heart rate abnormalities, such as a sinusoidal pattern; fetal death due to exsanguination can occur within minutes. In monochorionic twin gestations, the perinatal mortality rate is high for both twins, even if the vasa previa is associated with only one twin, due to the presence of placental vascular anastomoses [ 31 ].

As with velamentous cord insertion, the membranous vessels are at risk of compression from the fetal presenting part since they are not protected by the structure of a normal umbilical cord. (See 'Clinical course' above.)

Pathology  — Pathological examination may reveal membranous vessels, but otherwise is not useful since the pathologist cannot determine the location of the placenta and cord in the uterus.

Diagnosis  — Prenatal diagnosis of vasa previa is based primarily on identification of membranous fetal vessels passing across the internal cervical os by real-time and color Doppler ultrasound ( image 3 ). (See 'Imaging' above.) In studies in which the investigators were specifically looking for velamentous cord insertion or vasa previa, sonography had high diagnostic sensitivity (78 percent [ 26 ]) and specificity (91 percent [ 22 ]). In cases of diagnostic uncertainty, magnetic resonance imaging can be used to clarify the ultrasound diagnosis [ 32-35 ].

In the absence of prenatal diagnosis, a clinical diagnosis of vasa previa should be suspected in the setting of vaginal bleeding that occurs upon rupture of the membranes and is accompanied by fetal heart rate abnormalities, particularly a sinusoidal pattern or bradycardia. Confirmation that the blood is fetal via Apt, Kleihauer-Betke tests, or other tests (Ogita, Londersloot) supports the diagnosis [ 36 ]; however, there is usually no time to wait for test results before performing an emergency cesarean delivery for fetal distress.

Differential diagnosis

Funic presentation  — A loop of umbilical cord lying over the cervical os can be mistaken for vasa previa. In contrast to vasa previa, the umbilical vessels in funic presentation are surrounded by Wharton’s jelly and can float away from the cervical os if the uterus is shaken or the patient is placed in knee-chest or Trendelenburg position.

Cervico-uterine vessels  — In vasa previa, pulsed Doppler will demonstrate a rate consistent with the fetal heart rate and thus distinguish blood flowing in fetal vessels from maternal blood flowing in cervical arteries or a marginal utero-placental vascular sinus.

Cervical varicosities are rare in pregnancy [ 37-39 ]. Like vasa previa, cervical varices can appear as sonolucent tubules in the area of the internal os with blood flow on Doppler imaging. However, the tubules do not pass across the os, are tortuous, and may be part of a venous plexus.

Amniotic band or chorioamniotic separation  — An amniotic band or chorioamniotic separation may create the appearance of a sonolucent structure crossing the cervical os; however, color Doppler will not demonstrate blood flow, thereby excluding the diagnosis of vasa previa.

Screening  — Transvaginal ultrasound examination to look for membranous vessels proximate to the cervical os is reasonable in high risk patients: women with a velamentous cord insertion; pregnancies conceived following use of assisted reproductive technologies (in vitro fertilization); women with second-trimester low-lying placentas, placenta previa (even if it has resolved), or bilobed or succenturiate lobe placentas in the lower uterine segment; and women with multiple gestations [ 40-42 ].

The benefits of antenatal diagnosis and targeted management were illustrated by a series that reported neonatal survival in 59 of 61 cases (97 percent) suspected antenatally, but only 41 of 94 cases (44 percent) without prenatally suspected diagnosis [ 42 ]. Surviving infants without prenatally suspected diagnosis had lower Apgar scores (mean five-minute Apgar score of 4 versus 9) and required more blood transfusions (59 versus 3 percent). The mean gestational age at delivery of survivors and non-survivors was 36.5 and 37.6 weeks, respectively, and mean gestational age at delivery in those with and without prenatal diagnosis was 34.9 and 38.2 weeks, respectively. However, these results may be biased since most of the data came from women who chose to register with the Vasa Previa Foundation's web site.

A decision-analytic model to estimate the lifetime incremental costs and benefits of screening for vasa previa concluded that universal transvaginal ultrasound screening of twin pregnancies had an incremental cost-effectiveness ratio (ICER) of $5488 per quality-adjusted life-years (QALY)-gained [ 41]. For singleton pregnancies with risk factors including low-lying placentas, pregnancy following in vitro fertilization, accessory placental lobes, or velamentous cord insertion, the ICER was $15,764 per QALY-gained. In contrast, universal transvaginal screening for vasa previa in singleton pregnancies was not cost effective ($579,164 per QALY).

Management  — There are no data from randomized trials on which to base recommendations. If vasa previa is identified on prenatal ultrasound examination, we begin twice weekly nonstress testing at 28 to 30 weeks of gestation to look for any evidence of cord compression. Because of the high risk of emergency preterm delivery, we and others [ 40 ] suggest administration of a course of betamethasone between 28 and 32 weeks of gestation and hospital admission between 30 and 32 weeks of gestation for more frequent fetal heart rate monitoring. We perform nonstress tests on our patients two to three times daily.

We deliver the fetus by emergency cesarean delivery if any of the following occur:

• Labor
• Premature rupture of membranes
• Repetitive variable decelerations refractory to tocolysis
• Vaginal bleeding accompanied by fetal tachycardia, a sinusoidal heart rate pattern, or evidence of pure fetal blood by Apt test or Kleihauer-Betke assessment

There are no high quality data on which to base a recommendation for optimal timing of scheduled delivery. We generally agree with the authors of large series and decision analyses who have recommended delivery at about 35 weeks of gestation, without assessment of fetal lung maturity [ 40,43 ].

SUMMARY AND RECOMMENDATIONS

• The prenatal diagnosis of velamentous cord insertion is based upon the presence of characteristic sonographic findings (splayed, membranous umbilical vessels with no Wharton’s jelly) at the placental umbilical cord insertion site. A definitive diagnosis of velamentous cord insertion is made by pathologic examination of the placenta, cord, and membranes after delivery. (See 'Ultrasound and gross examination' above and 'Diagnosis'above.)
• The prenatal diagnosis of vasa previa is based upon characteristic sonographic findings (membranous vessels that cross the internal cervical os). In the absence of prenatal sonographic diagnosis, a clinical diagnosis of vasa previa should be suspected in the setting of vaginal bleeding that occurs upon rupture of the membranes and is accompanied by fetal heart rate abnormalities, particularly a sinusoidal pattern or bradycardia. Fetal exsanguination can occur within minutes. (See 'Imaging' above and 'Clinical course' above and 'Diagnosis' above.)
• In the second trimester, transvaginal ultrasound examination to look for vasa previa is reasonable in pregnancies with: velamentous cord insertion; conceived following use of assisted reproductive technologies; low-lying placentas, placenta previa, or bilobed or succenturiate lobe placentas in the lower uterine segment; and multiple gestations. We suggest both ultrasound examination and color Doppler of the umbilical cord insertion site. (See 'Screening' above.)

Velamentous umbilical cord:  

• The vessels in a velamentous umbilical cord are at increased risk of compression compared to a normal cord. We suggest fetal heart rate monitoring to detect compression of cord vessels beginning at 36 weeks of gestation. (See 'Management' above.)
• There is no evidence that induction of labor or scheduled cesarean delivery improve the outcome of pregnancies complicated by velamentous cord insertion without vasa previa. We continuously monitor the fetal heart rate during labor and exercise caution when exerting traction on the umbilical cord after birth. (See 'Management' above.)

Vasa previa:

• The frequency and severity of cord compression may be higher for vasa previa than velamentous umbilical cord (without vasa previa); therefore, we monitor pregnancies with vasa previa more closely. We suggest twice weekly nonstress testing beginning at 28 to 30 weeks, with hospitalization at 30 to 32 weeks of gestation for more frequent monitoring to detect early evidence of cord compression. Hospitalization also facilitates the ability to perform an emergency cesarean delivery in the event of premature rupture of membranes or preterm labor. (See 'Management' above.)
• For pregnancies with vasa previa, we suggest scheduled delivery at about 35 weeks of gestation ( Grade 2C ). (See 'Management' above.)

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