What is a vasa previa?
Vasa previa occurs when fetal blood vessels that are 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 that run 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 that connect the 2 placental lobes course over or near the cervix. Although there are no standardized criteria for how close the fetal vessels must be to the internal os to constitute vasa previa, a threshold of 2 cm has been proposed. In 1 series, all emergent deliveries with vasa previa had a fetal vessel within 2 cm of the cervical os.
What are the clinical implications of vasa previa?
Approximately 1 per 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 could 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 that are complicated by vasa previa is <10%, largely owing to improved prenatal diagnosis with ultrasound scanning. The largest study of pregnancy outcomes to date is a retrospective review of 155 cases from a patient-support website (n = 87) and data from 6 different medical centers (n = 68). This study found the survival rate for prenatally diagnosed vasa previa to be 97.6%, compared with 43.6% with intrapartum or postnatal diagnosis. Selection bias most likely contributed in part to these survival differences because of patient self-reporting of postnatal diagnoses that were complicated by adverse outcomes. In cases with prenatal diagnosis, 3.4% of newborn infants required transfusion, compared with 58.8% in those infants without prenatal diagnosis.
In a series of 56 cases with prenatal diagnosis, preterm bleeding occurred in 42% of cases with emergent delivery that occurred in 4.1% of singleton and 28.6% twin pregnancies. In another large series 28% of cases with prenatal diagnosis were delivered emergently.
What are the clinical implications of vasa previa?
Approximately 1 per 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 could 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 that are complicated by vasa previa is <10%, largely owing to improved prenatal diagnosis with ultrasound scanning. The largest study of pregnancy outcomes to date is a retrospective review of 155 cases from a patient-support website (n = 87) and data from 6 different medical centers (n = 68). This study found the survival rate for prenatally diagnosed vasa previa to be 97.6%, compared with 43.6% with intrapartum or postnatal diagnosis. Selection bias most likely contributed in part to these survival differences because of patient self-reporting of postnatal diagnoses that were complicated by adverse outcomes. In cases with prenatal diagnosis, 3.4% of newborn infants required transfusion, compared with 58.8% in those infants without prenatal diagnosis.
In a series of 56 cases with prenatal diagnosis, preterm bleeding occurred in 42% of cases with emergent delivery that occurred in 4.1% of singleton and 28.6% twin pregnancies. In another large series 28% of cases with prenatal diagnosis were delivered emergently.
What are risk factors for vasa previa?
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 the second-trimester ultrasound scan. In addition, 20% with vasa previa have a low-lying placenta at delivery.
Another risk factor that has been identified consistently 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 multple. An increased prevalence of vasa previa has also been described with multiple gestations. However, in many cases, this occurred in the setting of in vitro fertilization. Thus, the risk appears to be more modest with spontaneous twins.
How is vasa previa diagnosed?
The diagnosis of vasa previa by ultrasound scanning was first reported in 1987. Routine ultrasound evaluation of the placenta and lower uterine segment permits detection of the majority of cases. In a recent systematic review of 8 series that included >400,000 pregnancies and 138 cases of vasa previa, the median detection rate was 93%, with a specificity 99%. Although it can be diagnosed antenatally by transvaginal ultrasound scanning, vasa previa can be missed even under optimal circumstances.
Prenatal diagnosis of vasa previa by ultrasound scanning is most often made at 18–26 weeks of gestation, and identification is less effective if the ultrasound examination was performed only in the third trimester. If diagnosed in the second trimester, approximately 20% of cases resolved before delivery.
The following algorithm is recommended to facilitate the diagnosis of vasa previa and applies to all pregnancies ( Figure 1 ).
- •
At the time of mid-trimester ultrasonography, the placental location and the relationship between the placenta and internal cervical os should be evaluated.
- •
The American Institute of Ultrasound in Medicine and the American College of Obstetricians and Gynecologists also recommend that the placental cord insertion site be documented when technically possible.
- •
A follow-up ultrasound should be performed at 32 weeks of gestation for women who were diagnosed with placenta previa or low-lying placenta at the mid-trimester ultrasound examination. Since placenta previa detected in the middle of the second trimester that later resolves and low-lying placenta, even it it later resolves, are associated with vasa previa and consequently high perinatal mortality rates, transvaginal ultrasonography with color and pulsed Doppler is recommended to rule out vasa previa. These recommendations are for asymptomatic women, an earlier ultrasound may be indicated in owmen who are bleeding.
- •
If vasa previa is suspected, transvaginal ultrasound scans with color and pulsed Doppler should be used 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 ( Figures 2 and 3 ). The course of the vessel should be evaluated carefully to visualize it within the membranes and to exclude other possible causes of a vessel in close proximity to the cervix, such as funic presentation, marginal vein, or venous sinus.
