How to screen for vasa previa

How to .…Practical advice on imaging-based techniques and investigations with accompanying slides and videoclips online ‘Vasa previa’ refers to a condition in which unprotected fetal vessels traverse the fetal membranes in the lower uterine segment or over the cervix. These vessels may rupture in labor or when spontaneous or artificial rupture of the membranes occurs, carrying a high risk of fetal death. Vasa previa can be identified by ultrasound examination. When it is identified prenatally and patients are delivered prior to the onset of labor or rupture of membranes, the outcome for the baby is typically excellent, with a 97–100% survival rate1-3. However, failure to identify vasa previa prenatally may result in catastrophic delivery events, including fetal or neonatal death, severe fetal anemia requiring transfusion and cerebral palsy in survivors4, 5. Overall, 1:1275–2500 pregnancies are affected by vasa previa6, 7; in pregnancies conceived by in-vitro fertilization (IVF), the incidence is particularly high, at approximately 1:2608. Risk factors for vasa previa include: IVF pregnancy, velamentous or marginal cord insertion into the placenta, resolving placenta previa, bilobed or succenturiate lobes of the placenta and third-trimester bleeding8. However, approximately 11% of cases have no risk factors9. In a decision and cost-effectiveness analysis, screening strategies, including: (1) screening all IVF pregnancies and (2) ultrasound screening for risk factors, were found to be appropriate for clinical practice10. Three variants of vasa previa have been described. Type I occurs when there is a velamentous cord insertion and vessels run between the umbilical cord insertion site, through the fetal membranes, and the placenta11. Patients are at increased risk for this type of vasa previa if there is a velamentous or marginal cord insertion in the lower uterine segment. In Type 2, the free vessels course through the membranes between two lobes of the placenta in the lower segment11, which can occur in pregnancies with a bilobed or succenturiate placenta. In Type 3, there are one or more large ‘boomerang’ vessels which run along the margin of the placenta, through the membranes12; this may occur in cases with resolving placenta previa. In all cases, vessels near the cervix can be either arterial or venous, and rupture of either due to normal labor events can be catastrophic for the fetus. There have been no clinical trials of different management protocols for patients identified with vasa previa. Most protocols suggest administration of steroids and delivery prior to the onset of labor, typically between 34 and 37 weeks of gestation. Some protocols include cervical-length screening. Hospitalization has also been recommended, especially in patients with risk factors for preterm delivery and in those who have had vaginal bleeding10, 13-15. Since some pregnancies with vasa previa may have no clear risk factors9 and vasa previa is relatively rare, a high index of suspicion is required when performing screening ultrasound scans. The International Society of Ultrasound in Obstetrics and Gynecology (ISUOG)16, American Institute of Ultrasound in Medicine17 and Royal College of Obstetricians and Gynaecologists18 currently do not recommend screening for vasa previa as part of the routine anatomic survey. However, screening for vasa previa is relatively easy to do, has been found to be effective in some studies6, 19 and may be cost-effective10. The most common ultrasound finding in cases of vasa previa is a velamentous or marginal cord insertion into the placenta. At the time of the midtrimester scan, up to 99% of all placental cord insertion sites can be identified with transabdominal imaging19, 20. The placental cord insertion site can also be identified in the first trimester of pregnancy at the time of the 11–13-week ultrasound examination19, although identification of a low-lying or velamentous cord insertion into the placenta at this time requires re-evaluation later in pregnancy6. Typically, it is not difficult to identify the placental cord insertion into the placenta with transabdominal grayscale imaging. When the placental cord insertion site is difficult to find, it is often velamentous or, when the placenta is posterior, located under the fetus (Figure 1). Color Doppler imaging is helpful to trace the umbilical cord to the placental end or to the uterine wall. If the placental cord insertion site cannot be evaluated confidently using a transabdominal approach, then transvaginal scanning is necessary to ensure that there are no vessels in the lower uterine segment. Evaluation of the placenta to ensure that it comprises a single placental mass should be done at every visit. Succenturiate or bilobed placentae may become easier to detect as the masses move apart with uterine growth and it is not unusual to identify a succenturiate placenta later in pregnancy when one was not suspected earlier. The finding of a succenturiate or bilobed placenta should prompt a search for the connecting vessels between the two lobes. Transvaginal scanning should be performed to ensure that none of the vessels is in the lower uterine segment. Typically, the edge of the placenta in relation to the internal os is evaluated with transabdominal grayscale imaging at each visit from the second-trimester anatomic survey onwards, to determine if there is placenta previa. When this investigation is being carried out, the lower uterine segment should be evaluated for presence of vessels with grayscale and, if available, color Doppler imaging. Vessels are typically first suspected on grayscale imaging, using either a transabdominal or transvaginal approach, as circular or linear hypoechoic structures in the lower uterine segment21. For teaching purposes, they may be more easily remembered as either ‘bubbles’ or ‘lines’ in the lower segment (Figures 2a and 3a). These structures should always prompt further evaluation, as vasa previa is very likely. It is important to confirm that the structures are indeed fetal vessels, which is achieved using color and pulsed wave (spectral) Doppler (Figures 2b and 3b). When there is a cluster of vessels in the lower uterine segment which seem to run in different directions, this is often an indication that there is a velamentous cord insertion at the center of these vessels. The vessels fan out from the cord insertion site, entering the placenta in several different places (Figure 4). When a velamentous cord insertion, a bilobed or succenturiate placenta, a resolving placenta previa or a low-lying placenta is identified, transvaginal imaging is needed. Likewise, if the cord insertion site cannot be identified or if there is doubt about whether there are vessels in the lower segment, transvaginal imaging should be performed to evaluate the lower segment. The location of the vessels and their distance from the internal os should be evaluated. Transvaginal scanning complements transabdominal scanning, as some vessels may be difficult to see if they run at a 90° angle to the transducer beam. Using transabdominal as well as transvaginal scanning enables these structures to be visualized from different angles, improving detection (Figure 5). Evaluation by sweeping the transducer from side to side through the entire lower uterine segment is mandatory, using both grayscale and color Doppler imaging (Videoclips S1 and S2). Pulsed wave (spectral) Doppler is helpful to determine if vessels are arterial or venous in origin (Figure 6). Fetal arterial vessels will have pulsations similar to the fetal heart rate. If placenta previa or marginal previa is identified, the patient should be evaluated later in pregnancy to confirm resolution of the previa and to ensure that there are no vessels in the lower uterine segment or on the margin of the placenta (Type-3 vasa previa). This is best done using a transvaginal approach. If the fetal head is in the pelvis, especially in late pregnancy, it must be dislodged gently to allow evaluation of the lower uterine segment. This is accomplished by gentle pressure downward in the patient's suprapubic region with the free (non-scanning) hand, which lifts the fetal shoulder and head upward22. Once the fetal head is out of the pelvis, the lower uterine segment can be evaluated with transvaginal imaging. It is helpful to have an assistant perform this manipulation and it is easier to do at 32 weeks than at 36 weeks, when the fetal head is likely to be more fully engaged. Due to growth of the lower uterine segment, vasa previa may resolve in up to 25% of patients in whom it is diagnosed in the second trimester23. It has been suggested that resolution should be diagnosed when vessels are more than 2 cm from the internal os; however, this distance may not prevent all cases of vessel rupture24. In our opinion, a more reasonable distance may be 5 cm from the internal os, but there is no definitive ‘safe’ distance, especially when the intervention under consideration is early Cesarean delivery, which must be weighed against the potential for fetal death without intervention25. Furthermore, there are no data to suggest whether patients with unprotected vessels that lie between 2 cm and 5 cm from the internal os may safely undergo labor and vaginal delivery. Since there have been no trials addressing this issue, informed consent is suggested. Repeat evaluation with transvaginal scanning of the lower uterine segment towards term is suggested. Funic presentations can sometimes be confused with vasa previa, especially if the cord is close to a uterine wall. If the patient coughs, or if the fetus moves, a free loop of cord will move when the amniotic fluid shifts, allowing the funic presentation to be differentiated from vasa previa. However, the entire length of the free loop should be examined to ensure that it does not insert into the lower segment, the placental cord insertion site should be identified and vasa previa should be excluded. This is best done using a transvaginal approach (Figure 7). Suspicion of vessels in the lower uterine segment on transabdominal ultrasound should be confirmed by transvaginal sonography. Some uterine artery varicosities, uterine vein varicosities, amniotic bands, chorioamniotic separation or placental edge sinuses may mimic vasa previa, resulting in a false-positive diagnosis26, 27. Color Doppler with pulsed wave Doppler can be helpful to clarify the diagnosis. If the vessel has an arterial signal corresponding to a normal fetal heart rate, the diagnosis of vasa previa is clear. For venous structures without arterial vessels nearby, it can be helpful to trace the suspected vessels to their origin15, repeat the evaluation or obtain a second opinion. While it may not be possible to identify all cases of vasa previa, identifying the placental cord insertion site, assessing the lower uterine segment for ‘bubbles’ and ‘lines’, determining the number of placental masses and carefully evaluating a resolving low-lying placenta or placenta previa will identify the vast majority of cases, thus improving the neonatal outcome. Data sharing not applicable to this article as no datasets were generated or analysed during the current study. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.