Vasa previa: easy to miss
2017-05-27

Vasa previa (VP) is a condition that can result in a devastating fetal outcome, especially if it is not diagnosed during the antepartum period. Failure to identify the condition has been associated with a neonatal survival rate of less than 50%; however, if an accurate diagnosis is made during the course of pregnancy, the survival rate increases to 97–99%1, 2. The diagnosis of VP on ultrasound is made if a fetal vessel is visualized coursing through the membranes close to or over the internal cervical os. Although the minimum safe distance has not yet been determined, it has been proposed that the distance between the internal os and the vessel should be less than 2 cm to constitute VP3. There is an ongoing debate regarding the need to screen routinely for VP by prenatal ultrasound. So far, this recommendation has not been implemented in most national guidelines, mainly due to the low prevalence of the condition (0.6 in 1000 pregnancies)4. Certain risk factors are associated with increased incidence of VP, such as presence of placenta previa in the second trimester, low-lying placenta, marginal or velamentous umbilical cord insertion, accessory placental lobe, advanced maternal age and assisted reproduction4. At least one of these risk factors is present in about 80–90% of VP cases2. Correct diagnosis of VP is best accomplished using transvaginal ultrasound with color Doppler. In a recent meta-analysis, Ruiter et al. showed that this approach allows the identification of all cases of VP in affected pregnancies with a false-positive rate of 0.2–1%5. Certain maternal and fetal factors can make the diagnosis of VP more challenging. For example, in cases in which the fetal head is applied firmly to the lower uterine segment, VP can be missed easily. In these cases, an attempt should be made to dislodge the head of the fetus to allow a proper examination of the lower uterine segment and the internal os. We present a case in which suspicion of VP was raised initially at 23 weeks' gestation due to detection of a low-lying placenta with a marginal placental cord insertion on transabdominal ultrasound (Figure 1a). A fetal vessel extending from the cord insertion to an accessory placental lobe was seen coursing over the internal cervical os (Figure 1b). The diagnosis of VP was confirmed by two- and three-dimensional transvaginal ultrasound. At 31 weeks' gestation, the fetal head was noted to be applied firmly to the lower uterine segment and the aberrant vessel could not be visualized readily on ultrasound (Figure 1c). However, after pushing the head upwards (Videoclip S1), visualization of the VP was achieved (Figure 1d). This case highlights the importance of careful examination of the lower uterine segment in order to ensure that a diagnosis of VP is not missed. It is worth noting that, as the fetal head is usually not low during the second trimester, visualization of the lower uterine segment is obstructed rarely by the fetal head at the time of the second-trimester anomaly scan, thus supporting the contention that screening for VP should be performed at about 20 weeks' gestation. 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.