The definition, epidemiology, natural history, diagnosis, and management of vasa previa, particularly cases diagnosed prenatally.
By the Society for Maternal-Fetal Medicine (SMFM); Rachel G Sinkey, MD; Anthony O Odibo, MD, MSCE; and Jodi Dashe, MD
Q: What is vasa previa?
A: Vasa previa occurs when fetal blood vessels unprotected by the umbilical cord or placenta run through the amniotic membranes and traverse the cervix. Two types of vasa previa have been described. Type I occurs when there is a velamentous cord insertion between the umbilical cord and placenta, and fetal vessels running freely within the amniotic membranes overlie the cervix or are in close proximity to it. Pregnancies with resolved placenta previa or low-lying placenta are at risk for Type I vasa previa.
Type II occurs when the placenta contains a succenturiate lobe or is multilobed (typically bilobed), and fetal vessels connecting the 2 placental lobes course over or near the cervix. Although no standardized criteria exist regarding how close the fetal vessels must be to the internal os to constitute vasa previa, a threshold of 2 cm has been proposed. In one series, all emergent deliveries with vasa previa had a fetal vessel within 2 cm of the cervical os.
Q: What are the clinical implications of vasa previa?
A: Approximately 1 in 2500 deliveries are complicated by vasa previa. If membranes rupture, these vessels may rupture, with resultant fetal hemorrhage, exsanguination, or even death. In addition, fetal asphyxia can occur if sufficient pressure is applied to vessel(s) overlying the cervix and circulation is compromised.
In most recent case series, the perinatal mortality rate for pregnancies complicated by vasa previa is less than 10%, largely owing to improved prenatal diagnosis with ultrasound. The largest study of pregnancy outcomes to date is a retrospective review of 155 cases; that study found the survival rate for prenatally diagnosed vasa previa to be 97.6%, compared with 43.6% with intrapartum or postnatal diagnosis.
In a recent series of 56 cases with prenatal diagnosis, preterm bleeding occurred in 42% of cases, with emergent delivery occurring in 4.1% of singleton and 28.6% of twin pregnancies. In another large series, 28% of cases with prenatal diagnosis were emergently delivered.
Q: What are risk factors for vasa previa?
A: The 2 major risk factors for vasa previa are velamentous cord insertion-which accounts for the majority of reported cases-and succenturiate placental lobe or bilobed placenta. Approximately 60% of women with vasa previa at delivery had a placenta previa or low-lying placenta identified during second-trimester ultrasound. In addition, 20% of patients with vasa previa have a low-lying placenta at delivery.
Another risk factor that has been consistently identified is in vitro fertilization, which may increase the risk for Type 1 vasa previa to approximately 1 in 250, regardless of whether the gestation is a singleton or a multiple. An increased prevalence of vasa previa has also been described with multiple gestations. However, in many cases this occurred in the setting of IVF. Thus the risk appears to be more modest with spontaneous twins.
Q: How is vasa previa diagnosed?
A: In a recent systematic review of 8 series that included more than 400,000 pregnancies and 138 cases of vasa previa, the median detection rate was 93%, with a specificity of 99%. Although it can be diagnosed antenatally by transvaginal ultrasound (TVS), vasa previa can be missed even under optimal circumstances.
Prenatal diagnosis of vasa previa by ultrasound is most often made between 18 weeks and 26 weeks’ gestation, and identification is less effective if the ultrasound examination was performed only in the third trimester. Twenty percent of cases of vasa previa diagnosed in the second trimester resolve prior to delivery.
The following is recommended to facilitate the diagnosis of vasa previa and applies to all pregnancies.
• At the time of midtrimester ultrasonography, the placental location and the relationship between the placenta and internal cervical os should be evaluated.
• If placenta previa is detected in the second trimester and subsequently resolves, or if a low-lying placenta is identified in either the second or third trimester, TVS is recommended at 28 to 32 weeks’ gestation, with color and pulsed Doppler to evaluate for vasa previa.
• If vasa previa is suspected, TVS with color and pulsed Doppler should be utilized to facilitate the diagnosis.
• The diagnosis of vasa previa is confirmed if an arterial vessel is visualized over the cervix, either directly overlying the internal os or in close proximity to it, and color Doppler demonstrates a rate consistent with the fetal heart rate. The course of the vessel should be carefully evaluated in order to visualize it within the membranes and to exclude other possible etiologies of a vessel in close proximity to the cervix, such as funic presentation, marginal vein, or venous sinus.
Q: How should a pregnancy with a diagnosis of vasa previa be managed?
A: The goal of management of vasa previa is to safely prolong pregnancy while avoiding potential complications and mortality related to rupture of membranes (ROM) or labor. Because vasa previa is rare, recommendations regarding management are based on observational data, decision analyses, and expert opinion. If indications do not develop by 28 to 32 weeks’ gestation, it is reasonable to consider treatment with antenatal corticosteroids at that point.
Antenatal hospitalization has been proposed for women with pregnancies complicated by vasa previa, beginning at 30 to 34 weeks’ gestation, and in one series, more than half of such individuals followed as outpatients subsequently required hospitalization for a complication. The purpose of hospitalization is to allow for closer surveillance for signs of labor and to perform cesarean delivery in a timely manner to avoid membrane rupture.
Q: How and when should a pregnancy complicated by vasa previa be delivered?
A: The ultimate goal is to deliver before ROM while minimizing the impact of iatrogenic prematurity. Amniocentesis is not recommended to evaluate fetal lung maturity because delaying delivery is not helpful or recommended if fetal lung maturity is not confirmed.
Optimal timing of cesarean delivery remains unknown. In the largest retrospective series, fetuses diagnosed prenatally had a 97% survival rate, and the mean gestational age at delivery was 34.9 +2.5 weeks’ gestation. Data from a decision analysis study suggested that delivery between 34 and 35 weeks balances the risk of premature ROM and subsequent fetal hemorrhage and death versus the risks of prematurity; the authors found no benefit to expectant management beyond 37 weeks. Based on available data, planned cesarean delivery for a prenatal diagnosis of vasa previa between 34 and 37 weeks’ gestation is reasonable.
Download a PDF of a patient education handout on vasa previa at contemporaryobgyn.net/SMFM-vasa-previa
Delivery of a vasa previa pregnancy should occur by cesarean at a center capable of providing immediate neonatal blood transfusion if needed. The surgical team should make the hysterotomy mindful of the location of the placenta and aberrant blood vessels. In the event that a fetal vessel has been inadvertently lacerated during delivery, immediate cord clamping is recommended to prevent fetal/neonatal blood loss.
Delayed clamping of the umbilical cord is not recommended. In selected cases, preparations for delivery should include immediate availability of O-negative blood, in case of delivery of severely anemic neonate.