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Velamentous umbilical cord insertion and vasa previa

Velamentous umbilical cord insertion and vasa previa
Literature review current through: Jan 2024.
This topic last updated: Aug 30, 2023.

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 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 Wharton's jelly does not surround and thereby protect the vessels, they are prone to compression and rupture, especially when they are located in the membranes covering the cervical os (ie, vasa previa). These complications increase the chances of perinatal mortality and morbidity.

This topic will discuss issues related to a velamentous umbilical cord and vasa previa. Other umbilical cord abnormalities are reviewed separately.

(See "Umbilical cord abnormalities: Prenatal diagnosis and management".)

(See "Single umbilical artery".)

(See "Gross examination of the placenta", section on 'Umbilical cord'.)

(See "The placental pathology report", section on 'Umbilical cord'.)

VELAMENTOUS UMBILICAL CORD INSERTION

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 where the normal cord ends and the placental insertion) is highly variable.

Prevalence — Velamentous insertion occurs in approximately 1 percent of singleton gestations [1], but as many as 15 percent of monochorionic twin gestations [2-4]. It is 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 common hypothesis is that the cord's initial insertion site is central, but this location progressively becomes peripheral when the umbilical cord is unable to follow the "migration" of a very low lying the placenta as one half actively proliferates toward the well-vascularized uterine fundus (trophotropism) and the other half involutes [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 normal 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 are often located in the dividing membranes. (See "Gross examination of the placenta", section on 'Umbilical cord'.)

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 1A). If the uterus is shaken by the ultrasonographer, these vessels remain immobile; in contrast, a loop of umbilical cord will move when the uterus is shaken [6]. Color Doppler imaging enhances identification of the vessels (image 1B).

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

Clinical course — The clinical course may be benign or complicated by rupture, kinking, or compression. These complications increase the chances of perinatal mortality and morbidity, including preterm birth and growth restriction [7,8].

Because the vessels are attached to the chorion, rupture of the fetal membranes may also 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 [9]. (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 membranous vessels close to or covering the cervix are at risk of compression as the presenting part descends during labor. 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 [8]. 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 [10,11].

In a meta-analysis that evaluated the association between placental implantation abnormalities and risk of preterm birth in singleton gestations, velamentous cord insertions were associated with a high preterm birth rate (37.5 percent) and increased perinatal risks, such as neonatal intensive care unit admissions, small for gestational age, and perinatal death [7]. The majority of included studies were descriptive, did not have a control group, and had small numbers of cases. However, subsequent larger studies have also reported an increased risk of adverse perinatal outcomes, including growth restriction and death, although the absolute risks for these outcomes were small in most of the studies [8,12-17].

In monochorionic twins, velamentous insertion has been associated with discordant fetal growth/selective fetal growth restriction and twin-twin transfusion syndrome [18].

Multiple gestation — The prevalence of velamentous cord insertion in dichorionic twins, monochorionic diamniotic twins without twin-twin transfusion syndrome (TTTS), and monochorionic diamniotic twins with TTTS was 7.6, 34.7, and 36.1, respectively, in a series of 1498 twin placentas examined ex vivo [19]. Velamentous cord insertion did not appear to increase the risk of adverse outcome (small for gestational age, severe birth weight discordance) in dichorionic twins, but did appear to increase these risks in monochronic diamniotic twins.

In contrast, another study of 941 twin pregnancies reported the prevalence of velamentous cord insertion diagnosed by ultrasound was similar in dichorionic and monochorionic diamniotic twins (5.8 and 7.8 percent, respectively) and velamentous cord insertion in twin pregnancies was not a significant risk factor for adverse perinatal outcomes, including twin-specific complications (selective growth restriction, twin-twin transfusion syndrome, birthweight discordance) [20].

Maternal course — The mother is also at risk for complications, such as an increased risk for undergoing manual removal of the placenta at vaginal birth (frequency 5.5 percent of cases in one study [16]) and cesarean birth [8].

Diagnosis

Prenatal – The prenatal diagnosis of velamentous insertion is based on the presence of characteristic sonographic findings of membranous umbilical vessels at the placental cord insertion site, which is usually marginal (image 1A-B). 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 [1,21]. Imaging the cord in multiple planes improves diagnostic accuracy (eg, the cord may be misdiagnosed as nonvelamentous in some planes). (See 'Ultrasound and gross examination' above.)

Postnatal – A definitive diagnosis is made by direct gross examination of the placenta, cord, and membranes. The normal cord ends several centimeters from the placenta, 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). (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 recommends that, in the second and third trimesters, the umbilical cord should be imaged and the number of vessels in the cord should be documented; the placental cord insertion site should also be documented when technically possible [22]. 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 [23].

It has been noted that detection of an umbilical cord insertion in the lower third of the uterus during the first trimester is predictive of an abnormal cord insertion at delivery, particularly a velamentous cord insertion, as well as some placental abnormalities [24,25].

Management — There are no data from large or controlled studies on which to base management recommendations, so management is based on small studies/case reports, expert opinion, and good clinical judgment. Given the risks described above, if ultrasound examination suggests the presence of a velamentous umbilical cord, we suggest the following conservative approach:

Detailed fetal anatomic survey, including evaluation for coexistent vasa previa.

Serial assessment of fetal growth every four to six weeks, beginning at 24 weeks of gestation. If growth restriction and/or oligohydramnios is present, manage as per routine for these disorders. (See "Fetal growth restriction: Evaluation" and "Oligohydramnios: Etiology, diagnosis, and management in singleton gestations".)

Serial assessment of the cord insertion site at the time of fetal growth ultrasounds. As the lower uterine segment expands and the placenta continues to develop, the velamentous cord insertion site may become located further away from the cervix, reducing the chances of vessel rupture.

Patient counseling to call their provider as soon as labor begins.

Birth by 40 weeks of gestation – There is no evidence that interventions such as late preterm induction of labor or scheduled cesarean birth improve outcome of pregnancies with velamentous cord insertion. In our opinion, affected pregnancies can be followed expectantly and allowed to labor spontaneously and give birth vaginally, in the absence of additional pregnancy complications necessitating a different approach. However, we suggest delivery for pregnancies that reach 40 weeks of gestation since decreasing amniotic fluid volume in the late term period may place the membranous vessels at increased risk of compression.

Continuous intrapartum fetal heart rate monitoring to identify signs of severe cord compression or vessel rupture.

No or very gentle traction on the umbilical cord after cord clamping to avoid avulsion, which could result in a retained placenta. (See "Retained placenta after vaginal birth", section on 'Management'.)

VASA PREVIA

Definition — In vasa previa, fetal blood vessels are present in the membranes covering the internal cervical os, unprotected by placental tissue or Wharton's jelly. There are three types:

Type 1: membranous vessels associated with a velamentous or marginal umbilical cord insertion [26]

Type 2: membranous vessels connecting the lobes of a bilobed placenta or the placenta and a succenturiate lobe [26]

Type 3: one or more large boomerang vessels that run through the membranes along the margin of the placenta, such as with a resolving placenta previa [27,28]

By convention, the presence of aberrant blood vessels within 2 cm of the internal os is considered to have similar implications as vessels actually covering the internal os [29-32]. However, the precise distance associated with an increased risk of rupture has not been determined and only limited data are available to support the 2 cm measurement [31]. Some authors have suggested that the definition include vessels 2 to 5 cm from the internal os because such vessels also may rupture during labor [33,34]. However, more data are needed before altering management of patients with aberrant vessels >2 and ≤5 cm from the internal os as no trials have evaluated whether labor and vaginal birth is less safe in this setting.

Prevalence — The prevalence of vasa previa has been reported as 1 in 1300 to 1 in 2500 deliveries [30,35,36] but is much higher in pregnancies conceived following use of assisted reproductive technologies (prevalence as high as 1 in 202) [37-40]. The prevalence is also increased with second-trimester low-lying placentas or placenta previa (even if resolved before birth), bilobed or succenturiate lobe placentas in the lower uterine segment, and multiple gestations [39,41].

Pathogenesis — Pathogenesis is unknown but is likely similar to that for velamentous cord insertion. Resolution of placenta previa or low-lying placenta may result in type 1 vasa previa. (See 'Pathogenesis' above and 'Definition' above.)

Risk factors — Risk factors for vasa previa are not independent and include [36,42]:

Velamentous cord insertion

Umbilical cord insertion in the lower third of the uterus at first-trimester ultrasound

Placenta previa or low-lying placenta on second-trimester ultrasound scan

Succenturiate placental lobe or bilobed placenta

Assisted reproductive technology, particularly in vitro fertilization

Multiple gestation

The placental location and the relationship between the placenta and internal cervical os should be evaluated carefully in these patients.

In a systematic review of predictive indicators of vasa previa, at least 83 percent of cases had one or more risk indicators, most commonly placenta previa, assisted conception, velamentous cord insertion, and bilobed placenta [43]. A second-trimester low-lying placenta or placenta previa remains a risk factor for vasa previa at delivery even if the low-lying placental location resolves [34].

Clinical features

Imaging — On ultrasound examination, vasa previa appears as a linear sonolucent area that passes over the internal os. Color Doppler flow and waveform analysis show umbilical artery or vein waveforms and confirms that the sonolucency is a blood vessel (image 2).

In over 80 percent of cases, vasa previa will be associated with a placenta that is a previa, low-lying, bilobed, or succenturiate and the cord insertion will be velamentous or marginal cord [30,43,44].

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

Clinical course — In the majority of cases, the vasa previa persists and is at risk for rupture upon spontaneous or iatrogenic rupture of the membranes; rarely, fetal bleeding occurs without membrane rupture. Bleeding usually rapidly results in hypotension, leading to fetal heart rate abnormalities, such as a sinusoidal pattern; fetal death due to exsanguination can occur within minutes. Survivors are at high risk for long-term neurodevelopmental impairment [34]. There is no loss of maternal blood.

In a minority of cases, suspected second-trimester vasa previa resolves over time (the velamentous vessels remain [velamentous insertion] but no longer appear to cross the cervix). However, if a fetal head low in the pelvis impedes assessment of the placental cord insertion site, the head should be balloted to obtain views of the non-compressed lower uterine segment, otherwise resolution of vasa previa cannot be assumed.

In a retrospective multicenter cohort study of all antenatally diagnosed cases of vasa previa (defined as overlying the internal os), 19 of the 136 cases (14 percent) resolved spontaneously at median estimated gestational age of 27 weeks (range 19 to 34 weeks) [45]. The odds ratio for resolution when vasa previa was identified before 24 weeks of gestation versus later in gestation was 7.9 (95% CI 2.1-29.4), after adjustment for confounding variables. (See 'Differential diagnosis' below.)

In another retrospective cohort study including 100 vasa previa cases, the rate of vasa previa resolution was 39 percent, with resolution at a mean gestational age of 28.6±4.7 weeks; however, the authors defined vasa previa as a fetal vessel within 2 cm of the internal cervical os on transvaginal sonography [31,46]. The likelihood of resolution was dependent on vessel distance from the internal os and the gestational age at the time of diagnosis.

The frequency of fetal growth restriction appears to be increased. In one study, type 1 vasa previa was associated with lower mean birth weight than type 2 vasa previa (2494 versus 3037 grams), and lower placental weight [47]. The authors hypothesized that growth inhibition was due to a combination of a primary placental developmental disorder and alterations of the umbilical circulation in the velamentous cord.

Pregnancies complicated by vasa previa are at increased risk for preterm birth [34]. The likelihood of complications prompting or necessitating emergency delivery increases substantially at 34 to 35 weeks [30]. In two series, 50 to 60 percent of pregnancies were delivered by planned cesarean birth, and the remainder were delivered emergently primarily because of contractions or labor, bleeding, nonreassuring fetal heart rate tracing, or other (eg, asymptomatic cervical shortening, preeclampsia) [30,48].

As with velamentous cord insertion, the membranous vessels are at risk of compression from descent of the fetal presenting part since they are not protected by the structure of a normal umbilical cord. Compression could lead to fetal asphyxia. However, perinatal mortality rates are low with prenatal diagnosis and appropriate management, and obviously higher among pregnancies diagnosed intrapartum or postpartum as a result of fetal complications. (See 'Screening' below.)

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 [49].

Diagnosis

Prenatal – Prenatal diagnosis of vasa previa is based on identification of membranous fetal vessels passing across or in close proximity (within 2 cm) to the internal cervical os by real-time transvaginal ultrasound examination with color Doppler (image 2) (see 'Imaging' above and 'Definition' above and 'Screening' below). In prospective studies in which the investigators were specifically looking for vasa previa, sonography plus color Doppler had high diagnostic sensitivity: 10 of 10 cases after 26 weeks [26] and 1 of 1 case at 18 to 20 weeks [50]; in each study, one additional case diagnosed prenatally could not be confirmed at delivery. In a 2015 systematic review including both prospective and retrospective studies, sensitivity was 100 percent in four of the seven studies, 90 to 93 percent in two studies, and 53 percent in one study [51]. Subsequent studies have also reported sensitivity >90 percent [52].

Magnetic resonance imaging can be used to clarify the ultrasound diagnosis if there is diagnostic uncertainty and confirmation will affect pregnancy management [32,52-54].

Several case reports have described use of three-dimensional (3D) ultrasound technology for the diagnosis of vasa previa and to determine the optimum site for hysterotomy at delivery [55-58]. Although technically feasible, use of 3D technology has not been proven superior to two-dimensional (2D) technology, as arterial and venous flow can be easily assessed with 2D technology alone. In addition, 3D technology is not universally available.

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. Historically, tests such as the Apt or Kleihauer-Betke test or other tests (Ogita, Londersloot) were recommended for confirmation of fetal blood [59]; however, typically there is no time to wait for test results before performing an emergency cesarean birth for fetal distress, thus these tests have no practical role diagnosis or management.

Postnatal – Pathologic 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.

Differential diagnosis

Funic (cord) 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. 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 and a funic presentation. (See "Umbilical cord prolapse", section on 'Pregnancies with funic (cord) presentation'.)

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 seen frequently in pregnancy complicated by placenta previa (9 of 51 cases in one series) [60-63]. 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

Candidates – Transabdominal sonographic evaluation of placental appearance and location, including the site of cord insertion, is a standard part of the 18 to 22 week obstetric ultrasound examination [64]. More advanced techniques: transvaginal ultrasonography and color and pulsed wave Doppler, are used selectively at this examination to better evaluate pregnancies with risk factors for or suspected vasa previa. Even though risk factors such as midtrimester placenta previa and a low-lying placenta may not persist later in pregnancy, such pregnancies remain at risk for vasa previa and should be rescreened. The Society for Maternal-Fetal Medicine recommends a follow-up transvaginal ultrasound at 32 weeks for pregnancies with a previously diagnosed placenta previa or a low-lying placenta, and evaluation for vasa previa at this examination even if placental location is normal [65]. Use of transvaginal ultrasound plus color Doppler at this examination would detect approximately two-thirds of vasa previa cases with a low false-positive rate [64].

Utility – The benefit of prenatal diagnosis of vasa previa was suggested by a meta-analysis comparing pregnancy outcomes with versus without prenatal diagnosis of the condition (21 studies, 683 pregnancies with vasa previa) [66]. In this analysis, prenatal diagnosis (presumably followed by appropriate management) was significantly associated with more favorable outcomes:

Pooled perinatal survival: 98.6 versus 72.1 percent

Intact perinatal survival: 96.7 versus 28.1 percent

Despite improvement in outcomes with prenatal diagnosis of vasa previa, unplanned birth prior to the scheduled delivery date is common, occurring in 46 of 122 patients (38 percent) in one review [67]. Although overall neonatal mortality for prenatally diagnosed vasa previa was <1 percent, unplanned birth was associated with a high rate of neonatal complications, most likely secondary to birth at an earlier gestational age.

Technique [33]

If there is a single placental mass, identify the cord insertion site to determine whether the cord enters the placenta directly. Identification of a velamentous cord inserting into the placenta in the lower uterine segment is a significant risk factor for vasa previa.

If there is a succenturiate or bilobed placenta, search for connecting vessels between the two lobes.

If the fetal head impedes assessment, it should be balloted away from the cervix to improve visualization of the cervix and non-compressed lower uterine segment.

If the insertion site cannot be identified or there is a succenturiate or bilobed placenta or velamentous insertion, perform transvaginal ultrasound of the lower uterine segment to identify vessels, which may appear as circular or linear hypoechoic structures. Adding color Doppler and pulsed wave Doppler is also helpful to ascertain both the presence, course, and type of vessels. If vessels are present, determine the distance between them and the internal os. Care must be taken to include a lateral sweep of the internal os, as blood vessels located laterally may not be visible in the midline.

Management — There are no high-quality data on which to base recommendations for optimal timing of antenatal corticosteroid administration, type and frequency of antepartum fetal monitoring, need for and timing of antepartum hospitalization, value of cervical length measurements, and timing of scheduled delivery. Our recommendations, and those of others, are based on data from retrospective case series, expert opinion, and good clinical judgment. Our approach to third trimester management is described below and in the algorithm (algorithm 1).

Antepartum

Confirm the diagnosis – If the diagnosis was made in the mid-second trimester, the diagnosis should be confirmed in the early third trimester.

Activity – Pelvic rest and avoiding high-impact activities is probably prudent, although neither a benefit nor harm has been demonstrated [34].

Fetal growth – Because the risk of growth restriction is increased, ultrasound examinations to evaluate fetal growth every four weeks is reasonable, beginning at 24 weeks [34].

Antenatal corticosteroids – We suggest administration of a course of betamethasone by 34 weeks of gestation. The optimum time is based on clinical judgment, with consideration of patient-specific risk factors for preterm birth within the next seven days (eg, past obstetric history, cervical length, uterine irritability, comorbid conditions).

Ambulatory versus inpatient monitoring – We suggest hospital admission for close fetal surveillance (eg, nonstress testing [NST] at least daily) between 30 and 34 weeks of gestation, especially in patients with additional risk factors for preterm birth or any vaginal bleeding. Ambulatory monitoring is also a reasonable option in a select group of patients, particularly those with a long closed cervix, no contractions or vaginal bleeding, no history of spontaneous preterm birth, and who live within about 15 minutes of the hospital [68-71].

Since cervical shortening may be predictive of the onset of labor, as well as the risk of emergency birth in patients with a vasa previa [72], some authors have suggested using cervical length measurements to help with individualized decision making regarding safety of outpatient management and timing of birth; however, this approach has not been validated [34,72-74].

While the authors of this topic recommend inpatient management with daily testing, if the patient is committed to outpatient management and meets the criteria noted above, we suggest twice weekly NSTs and discuss with the patient that this is unlikely to prevent morbidity and mortality resulting from acute cord accidents.

In a meta-analysis of prenatally diagnosed vasa previa cases (1000 singletons, 109 twins), an institutional policy of routine hospitalization was associated with a reduction in neonatal transfusion (0.9 versus 6.0 percent) and a reduction in the perinatal mortality rate in twin pregnancies (0 versus 25 percent) but not in singleton pregnancies (0 versus 0.5 percent) [75]. Short cervical length (variously defined) had sensitivity of only 45 percent for predicting unscheduled birth during the next week.

Investigational interventions – Case reports have described use of fetoscopic laser ablation for treatment of type 2 vasa previa with variable outcomes [76,77]. While preliminary results are promising [78], this procedure should be considered investigational.

Route of delivery — Pregnancies with vasa previa should be delivered by cesarean [64]. Labor and vaginal birth can result in vasa previa compression and rupture and fetal exsanguination. (See 'Clinical course' above.)  

Vasa previa alone is not an indication for classical hysterotomy.

Ideally, the hysterotomy should avoid aberrant blood vessels. If a fetal vessel is lacerated inadvertently during birth, the cord should be clamped immediately to prevent fetal/neonatal blood loss [65].

Delivery timing — Expectant management is reasonable prior to 34 weeks as long as there are no worse than mild, intermittent variable decelerations on an otherwise reassuring nonstress test and preterm birth does not appear to be imminent. However, if the variable decelerations are persistent, the NST is nonreactive, or preterm birth appears to be imminent, then cesarean birth is usually indicated. The goal is to deliver the fetus before rupture of membranes or onset of labor while minimizing the sequelae accruing from iatrogenic preterm birth. (See 'Emergency cesarean' below.)

In a meta-analysis of prenatally diagnosed vasa previa cases (1000 singletons, 109 twins), the perinatal mortality rate attributable to vasa previa was 1.1 percent and all of the perinatal deaths occurred with unscheduled deliveries [75]. 

Emergency cesarean — We perform emergency cesarean birth if any of the following occur:

Labor.

Prelabor rupture of membranes.

Nonreactive nonstress test or persistent variable decelerations

Vaginal bleeding accompanied by fetal tachycardia, a sinusoidal heart rate pattern, or evidence of pure fetal blood by Apt test or Kleihauer-Betke test.

Type O negative blood should be available for emergency transfusion of a severely anemic newborn, when clinically indicated.

Scheduled cesarean — We agree with the authors of a decision analysis who recommended delivery at 34+0/7 to 35+6/7 weeks of gestation [79]; the precise timing within this range depends on clinician judgment of patient-specific factors. Birth at this gestational age achieved balance between the risk of perinatal death and the risks of mortality and morbidity related to preterm birth. ACOG and SMFM concluded planned cesarean birth because of vasa previa is reasonable at 34+0 to 37+0 weeks of gestation [65,80]. The Obstetrix Collaborative Research Network recommended planned birth at 33 to 34 weeks of gestation [73]. A decision-analytic model concluded the optimum time for delivery was 36 weeks, when weighing the risks of intrapartum mortality against gestational age neonatal complications [81]. A systematic review concluded that prolonging pregnancies until 36 weeks of gestation seemed to be safe and beneficial in otherwise uncomplicated pregnancies [82]. A clinical expert series recommended delivery at 36 to 37 weeks in asymptomatic patients without risk factors [34].

Multiple gestations – There is no clear evidence to recommend a different approach in twin pregnancies with vasa previa. Earlier scheduled birth at 32+0 or 33+0 weeks is reasonable if imminent birth seems likely because of a short cervix (<25 to 30 mm [42]) or threatened preterm labor.

In a meta-analysis of prenatally diagnosed vasa previa cases (1000 singletons, 109 twins), twin pregnancies were more likely to undergo unscheduled delivery (56.4 versus 35.1 percent) and deliver earlier (33.2 versus 35.1 weeks) [75]. The perinatal mortality rate in twin pregnancies was markedly higher than that in singleton pregnancies (9.2 versus 0.2 percent) and accounted for 80 percent of overall mortality although comprising 9.8 percent of births.

SUMMARY AND RECOMMENDATIONS

Velamentous umbilical cord

Diagnosis – 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.)

Management – The vessels in a velamentous umbilical cord are at increased risk of compression compared with a normal cord. Therefore:

Antepartum fetal surveillance – We suggest serial assessment of fetal growth every four to six weeks. If growth restriction and/or oligohydramnios is present, manage as per routine for these disorders. (See 'Management' above.)

Delivery timing – There is no evidence that induction of labor or scheduled cesarean birth improves the outcome of pregnancies complicated by velamentous cord insertion without vasa previa. However, we suggest delivery for pregnancies that reach 40 weeks of gestation since there is minimal neonatal morbidity at this gestational age and decreasing amniotic fluid volume in the late term period may place the membranous vessels at increased risk of compression. (See 'Management' above.)

Intrapartum care – 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

Diagnosis – The prenatal diagnosis of vasa previa is based upon characteristic sonographic findings (membranous vessels that cross or are in close proximity [within 2 cm] to 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.)

Risk factors – Risk factors for vasa previa include velamentous cord insertion, umbilical cord insertion in the lower part of the uterus at first-trimester ultrasound, placenta previa or low-lying placenta on second-trimester ultrasound scan, succenturiate placental lobe or bilobed placenta, in vitro fertilization, and multiple gestation. (See 'Risk factors' above.)

Screening – In patients with risk factors for vasa previa, the placental location and the relationship between the placenta and internal cervical os should be evaluated carefully using transvaginal sonography and Color Doppler. Patients with placenta previa (even if resolved) or a low lying placenta on midtrimester ultrasound examination should undergo transvaginal ultrasound with color and pulsed wave Doppler at 32 weeks of gestation to screen for vasa previa. (See 'Screening' above.)

Management – Our approach is shown in the algorithm (algorithm 1). For singleton pregnancies with uncomplicated vasa previa, we suggest scheduled delivery at 34+0 to 35+6 weeks of gestation (Grade 2C).

There is no clear evidence to recommend a different approach in twin pregnancies with vasa previa. Earlier scheduled birth at 32+0 or 33+0 weeks is reasonable if imminent birth seems likely. (See 'Delivery timing' above.)

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Topic 6807 Version 53.0

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