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Introduction

Vasa praevia is a rare disorder of placentation associated with an estimated prevalence between 1 in 2000 and 1 in 5000 pregnancies^(1-7). The reported incidence of vasa praevia varies, however, based on the method of conception (between 1 in 513 and 1 in 6000 cases in naturally conceived pregnancies, to 1 in 293 cases in artificially conceived pregnancies)^(8,9). It has been associated with a high perinatal mortality rate of approximately 60% if undiagnosed prenatally and managed appropriately⁽³⁾, carrying additional maternal risks associated with undergoing emergency caesarean section^(10-12).

Antenatal diagnosis of vasa praevia via ultrasound scanning has allowed for a significant drop in perinatal mortality (most recent case series suggesting a mortality rate of less than 10%), compared with intrapartum or postpartum diagnosis (which have reported survival rates of 43.6%). Fewer cases of blood transfusions in newborn infants with prenatally diagnosed vasa praevia have been reported (approximately 3%), compared with newborns diagnosed intra- or postnatally (approximately 60%)⁽¹²⁾.

Objectives

The focus of this paper is to evaluate the role of ul­tra­sound imaging in the diagnosis of vasa praevia, as well as to describe the etiology, the various clinical presentations, and the management strategies for this condition.

Materials and method

We searched PubMed, MEDLINE and the Coch­rane Library using appropriate variables (e.g., “vasa prae­via”, “placenta praevia”, “velamentous cord inser­tion”, “succenturiate”, “bilobed placenta”), along with the medical literature related to vasa praevia up to January 2021. We selected randomized control trials, observational studies, systematic reviews and four national guidelines developed by the Society for Mater­nal-Fetal Medicine, the Royal College of Ob­ste­trician and Gynecologists, the Royal Australian and New Zealand College of Obstetricians and Gynecologists and by the Society of Obstetricians and Gynecologists of Canada. The diagnostic and management strategies recommended in medical literature were compared and reviewed.

Risk factors and complications

Vasa praevia is an uncommon obstetric condition in which the fetal umbilical vessels, unsupported by the umbilical cord or placental tissue, cross the amniotic membranes above the internal os of the cervix and below the fetal presenting part^(10,12,13). Abnormal insertion of the umbilical cord (velamentous cord insertion or insertion in the lower third uterine part diagnosed at the first-trimester ultrasound^(14-16)), placental anomalies (placenta praevia, bilobed or succenturiate), multiple pregnancies or pregnancies obtained via assisted reproduction methods are just a few of the potential risk factors for vasa praevia^{(9,17-19,21,22)}.

Vasa praevia has a high risk of rupture in both active and augmented labor (especially when amniotomy is performed), leading to fetal hemorrhage, rapid exsanguination, or even death. During active labor, fetal blood vessels may be compressed, leading to fetal asphyxia⁽¹⁷⁾.

Vasa praevia classification

Vasa praevia has been classified into two main types⁽¹³⁾. In type I vasa praevia, the umbilical cord has a velamentous insertion and fetal blood vessels overlie the internal os, or are located near it. Thus, patients with low-lying placenta or resolved placenta praevia are at risk. Type II occurs when succenturiate or additional placental lobes, connected by the fetal umbilical vessels located over or near the cervix, have been identified⁽²³⁾.

In order to facilitate the diagnosis of vasa praevia using ultrasound examination, a consensus has been reached regarding the distance of the fetal vessels from the internal os, with a proposed threshold of no more than 2 cm^(24-26).

Antenatal diagnosis

The antenatal diagnosis of vasa praevia is generally made around 18 to 24 weeks of gestation by routine ultrasound scanning^(27,28), but needs to be confirmed by a follow-up ultrasound evaluation during the third trimester (at 30 to 32 weeks of gestation)^(3,24,29). The ultrasound diagnosis of vasa praevia during the second trimester has a reported estimated detection rate of 93%, 99% specificity⁽³⁰⁾ and a 20% rate of cases resolved before delivery^(25,31). If vasa praevia is first discovered in the third trimester, ultrasound examination has proven to be much less effective⁽²²⁾. Zang et al., in 2019, proved that accurate antenatal diagnosis of vasa praevia can be achieved by a two-stage screening strategy. In the first trimester, patients with velamentous cord insertion at the inferior pole of the placenta and second trimester patients with low-lying placenta must be classified as high-risk for vasa praevia and transvaginal ultrasound offered, specifically searching for vasa praevia at the time of the mid-trimester anomaly scan. The recommended strategy is to use color Doppler to diagnose or exclude vasa praevia by identifying vessels within 5 cm of the internal os⁽³²⁾.

The screening for vasa praevia can also be stratified ac­cor­ding to risk factors: velamentous umbilical cord in­ser­tion, succenturiate or placenta with accessory lobes, IVF preg­nancy, multifetal pregnancies^{(9,17-19,21,22)}. If, during the transabdominal scan, vasa praevia is sus­pected, trans­vaginal ultrasound evaluation using color and pulsed wave Doppler should be performed to confirm or to exclude the diagnosis. The presence of an arterial ves­sel in close proximity or directly over the internal cer­vi­cal os, with a blood flow rate matching the fetal heart rate, confirms the existence of vasa praevia^(33-35). When technically possible, it is important to establish the course and the existence of the fetal vessels within the membranes, in order to rule out other conditions (such as venous sinus, funic presentation or marginal vein) that may explain the presence of a vessel near the cervix (Figure 1)⁽³⁶⁾. The importance of cervical-length ultrasound screening is still unknown⁽³⁷⁾.
 

Intrapartum diagnosis

The detection of the pulsating fetal vessels in close proximity to the internal os via vaginal examination during early labor, vaginal bleeding or signs of acute fetal compromise following the spontaneous rupture of the membranes or amniotomy (e.g., sinusoidal fetal heart rate tracing or sudden onset fetal bradycardia) facilitate the intrapartum diagnosis of vasa praevia⁽³⁸⁾.

Management

The prenatal diagnostic rate of vasa praevia is ap­pro­xi­mately 98%^(30,39) and it provides the improvement of neonatal and maternal outcomes due to the ad­mi­nis­tra­tion of corticosteroids timed between 28 to 32 weeks of gestation for fetal lung maturation, and elective pre­term hospitalization timed between 30 to 34 weeks, based on individual factors, such as: history of pre­term birth, preterm contractions, vaginal bleeding, dis­tance from hospital and planned delivery via elective cae­sarean section prior to spontaneous rupture of membranes^(27,28) timed between 34 to 37 weeks of gestation (in asymp­to­matic women)^(40,41), in a center capable of providing im­me­diate neonatal blood transfusion (using type O ne­ga­tive blood) and pediatric support.

The American College of Obstetricians and Gyne­co­lo­gists and the American Institute of Ultrasound in Medicine have recommended an algorithm for the manage­ment of vasa praevia⁽⁴²⁾. Placental location, its distance from the internal cervical os and the placenta cord insertion site should be examined in the mid-trimester ultrasonography^(30,43,44). If no anomalies have been detected, routine care should be applied⁽³⁸⁾.

If placental anomalies have been detected (low-lying placenta or placenta praevia identified during the middle of the second trimester) in asymptomatic patients, a follow-up ultrasound scan is recommended around 30 to 32 weeks of gestation^(3,24,29), since these conditions, even though resolved, are often associated with vasa praevia⁽³⁸⁾. In order to rule out vasa praevia, trans­­ab­dominal, coupled with transvaginal ultra­so­no­­graphy with color and pulsed wave Doppler are recom­mended. If patients are symptomatic or present with vaginal bleeding, ultrasound evaluation may be indicated earlier. If vasa praevia and a cervical length of less than 25 mm are identified, hospitalization and corticosteroid administration should be conducted with planned delivery via elective caesarean section at 35 to 36 weeks of gestation. If placenta praevia is diagnosed, without vasa praevia, elective delivery shall be carried out at 37 weeks of gestation^(4,36). In cases when vaginal bleeding or premature rupture of membranes occur in patients antenatally diagnosed with vasa praevia, the admission to the birthing unit and continuous electronic fetal heart rate monitorization are recommended. When technically possible and if time permits, rapid tests for fetal hemoglobin should be conducted. Urgent caesarean section is recommended if abnormalities are detected during the fetal heart rate monitorization or in the fetal biochemical test^(38,45-51).

There is no conclusive data regarding the role of trans­vaginal ultrasound cervical length measurements and cerclage⁽³⁷⁾. The 2017 prospective population-based cohort study suggests that outpatient care is feasible and associated with excellent outcomes if vaginal ble­eding or preterm contractions are absent and if there is no evidence of cervical shortening⁽⁵²⁾. The weekly measure­ment of cervical length is recommended.

The preoperative evaluation of the course of the fetal vessels using color Doppler ultrasonography lowers the risk of intraoperative vessel laceration. If, however, laceration has occurred, immediate cord clamping should be performed, in order to limit the fetal/neonatal blood loss^(38,49).

No consensus has been reached regarding the optimal time for performing the caesarean section⁽⁵³⁾. The delivery between 34 to 37 weeks of gestation limits the risks of iatrogenic preterm delivery and complications regarding the onset of preterm labor^(12,54).

Discussion

Prenatal screening for placental cord insertion, placenta praevia or low-lying placenta, multifetal or IVF pregnancies, succenturiate or bilobed placenta or low cord insertion (in the first trimester) facilitate the timely diagnosis of vasa praevia^(12,24). The screening for vasa praevia is recommended during the mid-trimester ultrasound^(27,28) (99.8% specificity, and 100% sensitivi­ty)⁽³⁰⁾. The novel strategy on a two-stage protocol of screening for vasa praevia suggests that the antenatal diag­nosis of vasa praevia and the appropriate monitoring and delivery can potentially reduce the overall rate of stillbirth by about 10%⁽³²⁾. According to medical lite­ra­ture, the universal transvaginal screening with color and pulse Doppler is not recommended or cost-effective. False positives may occur when membrane separation or marginal placental sinuses are present, or when there is an umbilical cord loop covering the cervix. False negatives may result from improper angle of insonation during the color Doppler examination⁽¹⁹⁾.

When risk factors are present, targeted ultrasound screenings are recommended. Even so, undiagnosed cases of vasa praevia still occur^(6,55).

The role of cervical length ultrasound screening in the management of vasa praevia and the outcome of outpatient follow-up versus hospitalization at 30-32 weeks of gestation must still be established by prospective multicentre studies. The optimal timing of delivery in pregnancies with vasa praevia must also be further studied in ensuing randomized controlled trials. Due to insufficient data being provided regarding the management of vasa praevia, the available guidelines are inconsistent in their recommendations and management strategies. Most often, vasa praevia is managed according to individual institutional policy.

Conclusions

Vasa praevia is a relatively rare obstetric condition associated with a high perinatal mortality rate. When diag­nosed antenatally (around 18 to 24 weeks of ges­ta­tion), excellent outcomes can be expected. Being a re­la­tively rare and underreported disorder, screening stra­tegies for prenatally detectable risk factors should be further developed in both singleton and multiple preg­nancies. It is necessary to establish if cervical-length ul­tra­sound screening in pregnancies diagnosed with vasa praevia is useful. The routine screening for the umbilical cord insertion site is strongly recommended and should be performed when technically possible^(42,56).

Based on national guidelines, observational data and expert opinion, the current recommendations include ultrasound screening for risk factors, the administration of prenatal corticosteroids at 28 to 32 weeks of gestation and the delivery by elective caesarean section at 34 to 37 weeks of gestation in asymptomatic women, earlier (34 to 35 weeks of gestation) if the patients are at high risk of preterm delivery, with a further recommendation of delivery around 32 to 34 weeks of gestation in twin pregnancies.  

Conflict of interests: The authors declare no con­flict of interests.