Vasa previa results from an anomaly of placentation, which is defined as the presence of an aberrant fetal vessels running through the membranes across the internal os of the cervix under the fetal presentation [1–3]. There are two types of vasa previa depending if the free vessel is connected to a velamentous cord (type I) or connected to a succenturiate or accessory lobe of the main placenta with (type II).
The vasa previa is unprotected by the Wharton jelly of the umbilical cord or the tissue of the placental chorionic plate and thus exposed to rupture during the spontaneous rupture of the membranes of an active labor or at amniotomy to induce or augment early labor. The classic presentation of undiagnosed vasa previa during labor is the presence of painless vaginal fresh bleeding. Because the total fetal blood circulating volume at term is around 80–100 ml/kg, the loss of what may appear to be a relatively small amount of blood can have major impact on the fetus and be rapidly fatal [4–6].
The incidence of vasa previa has been reported to range between in one in 500 to one in 5000 pregnancies [1–6]. Similarly, to other disorders of placentation, that is, placenta previa and placenta accreta spectrum, there has been mounting clinical evidence over the last decade that prenatal diagnosis of vasa previa can have a direct impact on perinatal outcome [6–8,9▪,10▪▪]. Although it has been estimated that around 150 fetuses die at birth due to vasa previa in the UK alone , there is still a lot of controversy regarding universal screening of vasa previa, mainly due limited prospective data available [12,13]. Recently, several epidemiological studies, expert reviews and systematic reviews and meta-analysis have indicated that pregnancies resulting from assisted reproduction technology (ART), presenting with a low-lying placenta, a velamentous umbilical cord insertion and multiple pregnancies are at higher risk of vasa previa [14–17,18▪▪,19▪▪]. These findings suggest that targeted screening for vasa previa might be cost-effective [10▪▪]. The present review presents the latest evidence on the epidemiology, prenatal diagnostic techniques and screening strategies, and management of vasa previa.
A recent systematic review of 13 studies (two prospective cohort studies, 10 retrospective cohort studies and one case–control study) and reporting on 569 410 women found a mean incidence of 0.60 per 100 pregnancies. The authors found that 83% of the 325 vasa previa cases reviewed had one or more risk factor [18▪▪]. The highest odds ratio (OR) was found for the presence of a velamentous cord insertion (VCI) [OR, 672; 95% confidence interval (CI), 112–4034] followed by placental morphological anomalies, that is, bilobed placenta and succenturiate placental lobes, conception by ART; second trimester low-lying placenta and multiple gestations (Table 1). These data were confirmed by the recent prospective population-based cohort study undertaken using the Australasian Maternity Outcomes Surveillance System and showing that 55 (95%) of the 58 women had at least one risk factor for vasa previa with VCI (62%) and low-lying placenta (60%) the most prevalent [19▪▪].
Pregnancies conceived by ART and in particular in-vitro fertilization (IVF) have consistently been associated with a higher incidence of abnormal cord insertion [20▪▪,21–25]. The incidence of vasa previa has been reported to be as high as one in 200 pregnancies conceived after ART and is likely to increase in the next decade with the increase use of ART worldwide [10▪▪]. It has been hypothesized that vasa previa could be due to the inadequate orientation of the IVF blastocyst during implantation or to a higher incidence of vanishing twins in IVF than in spontaneous twins [16,20▪▪].
Our recent case series and systematic review has indicated a total incidence of 11.0% of vasa previa in twin pregnancies [20▪▪]. Like for singleton pregnancies, a VCI was the most common additional ultrasound findings in twins presenting with vasa previa. The incidence of succenturiate lobes and other morphological placental anomalies is higher in twin pregnancies [1,17,24,26]. A VCI of one of the umbilical cords is eight times more common in twins than in singletons . The OR for vasa previa in pregnancies presenting with a bilobate placenta or with succenturiate lobe has been evaluated to be 22.1 in twin pregnancies . Five of the seven cases in our series and five out of the eight cases described in detail in the literature presented with a VCI [20▪▪]. One cases in our series and three in the literature also presented with a low-lying placenta.
Gianopoulos et al. were the first to report in 1987 a case of vasa previa diagnosed by ultrasound at 25 weeks of gestation in a pregnancy presenting with a low-lying placenta and a VCI on a succenturiate lobe . Multiple case reports and small retrospective cohort series were published over the last three decades. The largest study published so far included 155 women with vasa previa confirmed at delivery . This study was the first to highlight the impact of prenatal diagnosis of vasa previa on perinatal outcome with a 97% survival rate in cases of diagnosed antenatally compared with only 44% when the diagnosis was made during delivery. Recent studies have shown that prenatal diagnosis in centers with prenatal diagnosis expertise can be associated with 100% neonatal survival [10▪▪,19▪▪].
Vasa previa was first diagnosed using gray-scale ultrasound imaging as parallel or circular echogenic lines near the internal cervical os . The advent of color Doppler imaging (CDI) has allowed a more precise diagnosis by demonstrating that these lines to be vessels with pulsed Doppler showing a fetal arterial or venous waveform (Fig. 1a–d). The combined used gray-scale and CDI is helpful in the differential diagnosis between vasa previa from placental marginal sinus (where pulsed Doppler reveals maternal vessel waveform) and from umbilical cord presentation (easily displaced by gentle pressure on the maternal abdomen) . New development in ultrasound imaging such as four-dimensional spatiotemporal image correlation (4D-STIC) may improve the diagnostic accuracy of vasa previa .
A systematic review of the diagnostic accuracy of ultrasound imaging in the diagnosis of vasa previa including two prospective and six prospective studies has found that the prenatal detection rates during second trimester ranging between 53 and 100% . Four of the studies used transvaginal ultrasound (TVS) as the primary method for vasa previa diagnosis whereas the remaining four used transabdominal ultrasound followed by TVS for confirmation of diagnosis. The only two prospective studies were using TVS CDI and have demonstrated high diagnostic rates with sensitivities of 100% and sensitivity of 99.0–99.8% [30,31]. Hasegawa et al. have shown that pregnancies at risk of vasa previa could be identified as early as 9 weeks of gestation . However, as the definitive placenta is only fully formed by 10–11 weeks of gestation making it impossible in many cases to predict the final position of the umbilical cord and to identify placental anomalies such as bilobed placenta, there appears to be little benefit in attempting to identify vasa previa during the first trimester. Overall, the prenatal diagnosis remains more effective around mid-gestation (18–26 weeks) than during the first or third trimester.
Universal screening for vasa previa is an attractive option to reduce perinatal mortality, but evidence in support of this strategy is unsupported by evidence. The incidence of vasa previa in the general population is lower than most fetal anomalies, its prevalence difficult to evaluate as not all placenta are examined at birth for this type of anomaly and the sensitivity of ultrasound of accurately screening for it difficult to assess as studies have been conducted in specialist centers and are mainly retrospective. A recent decision and cost-effectiveness analysis comparing four screening strategies for the prenatal screening of vasa previa in singleton pregnancies has shown that that screening pregnancies conceived by IVF is the most cost-effective strategy with an incremental cost effectiveness ratio of $29 186.50/quality adjusted life years [33▪▪].
Identification of the placental cord insertion during at the mid-gestation ultrasound examination to exclude a VCI which is the main risk factor for vasa previa, is easy and accurate, takes less than 1 min and requires no additional scanning skills for a trained sonographer (Fig. 2). However, there is no recommendation for routine screening for vasa previa as part of the complete ultrasound examination, and ultrasound diagnosis of vasa previa is not routinely taught during ultrasound training courses. We have recently shown that the implementation of standardized prenatal targeted scanning protocols for pregnant women with risk factors for vasa previa improves the prenatal diagnosis and thus the perinatal outcome. Our study included 51 cases with confirmed vasa previa at delivery and the prenatal detection rate increased after implementation of the screening protocol [nine of 18 cases (50%) vs. 29 of 33 cases (87.9%)]. Accurate prenatal diagnosis was shown to reduce the risks of perinatal complications mainly low Apgar scores and improves umbilical cord pH and hemoglobin levels for newborns with vasa previa [10▪▪].
There is currently no clear consensus on the optimal surveillance strategy including the need for antepartum hospitalization, timing of corticosteroids administration, and the value of transvaginal cervical length measurements. There are so far no randomized control trials to support any of the recommendation presented below. Overall management strategies are empirical and based on not being able to predict the optimal timing of delivery for individual women.
Recent national guidelines and expert reviews have recommended scheduled cesarean section of all asymptomatic women presenting with vasa previa between 34 and 36 weeks’ gestation [3,4,33▪▪,34▪,35]. A decision analysis comparing 11 strategies for delivery timing in a patient with vasa previa found that scheduled delivery at 34 weeks of gestation was the preferred strategy and resulted in the highest quality-adjusted life-years under the base-case assumptions . The authors also found no benefit to expectant management beyond 36 weeks of gestation and amniocentesis for verification of fetal lung maturity does not improve outcomes.
There is also is weak evidence based on low-quality data supporting systematic antenatal hospitalization in a unit with neonatal facilities from 30 to 32 weeks of gestation [33▪▪]. The purpose of hospitalization is to allow for closer surveillance for signs of labor, corticosteroids administration to enhance fetal lung maturation, and a timelier performance of cesarean delivery before labor and/or membrane rupture. The data of the recent prospective population-based cohort study using the Australasian Maternity Outcomes Surveillance System found no difference in perinatal outcome when vasa previa was diagnosed prenatally between women who were hospitalized compared to those with no antenatal hospitalization [19▪▪].
Sonographic measurements of cervical length have been shown to be useful in predicting and managing women at risk for preterm delivery , as well as predicting the risk of emergent cesarean delivery in placenta previa [38,39]. The role of TVS cervical length measurements in the management of pregnancies with vasa previa has only been evaluated recently. In a retrospective case–control study of 29 singleton pregnancies diagnosed with of vasa previa on ultrasound in the second, we found that the rate of cervical length shortening is significantly slower for women with elective cesarean delivery compared with those needing an emergent caesarean section [40▪]. For each additional millimeter-per-week decrease in cervical length, the odds of emergency cesarean delivery were increased by 6.50 (95% CI 1.02–41.20). Our data suggest that serial cervical length measurements should start from 26 to 28 weeks. If the cervix is shorter or shortening more rapidly, the patient should be hospitalized and administered corticosteroids to enhance fetal lung maturation. The cesarean delivery could be planned by section at 34–36 weeks if the patient remains asymptomatic (Fig. 3).
Like singletons, twin pregnancies presenting with vasa previa have very poor perinatal outcomes if the diagnosis is not made before the onset of labor. Twin pregnancies are at higher risks of preterm delivery and a recent retrospective cohort study of 441 women with twin pregnancies has found that integration of serial measurements of cervical length using a stepwise algorithm in asymptomatic women with twin gestations can improve the detection of women at risk of preterm birth . To minimize the impact of prematurity, twins diagnosed with vasa previa with uncomplicated antenatal course and stable cervical length, could be scheduled for caesarean delivery at 36 weeks. However, the data on cervical length from our series and from the literature suggest that a cervical length at least 30 mm could be a better limit to allow for conservative management in women with twins and vasa previa. The role of cervical length measurements in the management of singleton s and twins presenting with vasa previa need to be prospectively evaluated by a multicentric study.
Prenatal diagnosis of vasa previa prevents fetal demise during delivery in both singleton and twin pregnancies. Although there is insufficient evidence to support a universal mid-gestation ultrasound screening for vasa previa, recent epidemiological data support the development of standardized ultrasound screening protocols targeting pregnancies at high-risk of vasa previa, that is, presenting with VCI, conceived with IVF, multiple pregnancy, presenting with low-lying placenta or placental shape anomalies. Serial measurements of cervical length in pregnancies diagnosed with vasa previa may play a role in the management pathways of both singletons and twins and provide individualized care in particular regarding hospitalization and timing corticosteroids administration and delivery.
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Conflicts of interest
There are no conflicts of interest.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
- ▪ of special interest
- ▪▪ of outstanding interest
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