Vasa previa occurs when the umbilical vessels course through the membranes within the lower uterine segment over the internal cervical os. There are several known risk factor, including a velamentous cord insertion, placenta previa, multiple gestations, and a bilobed placenta. Because these vessels contain fetal blood, prenatal diagnosis is important and significantly decreases perinatal morbidity and mortality.
Keywordsvelamentous cord insertion, vasa previa, third-trimester bleeding
Vasa previa is a condition in which the umbilical vessels course through the membranes within the lower uterine segment and traverse over, or near, the endocervical os. These vessels can rupture spontaneously without rupture of the membranes, and result in fetal exsanguination and demise. In addition, since the vessels are not protected by the umbilical cord, they are at increased risk of cord compression.
Vasa previa occurs when the fetal vessels located over the internal cervical os are surrounded only by membranes without Wharton’s jelly.
Prevalence and Epidemiology
The prevalence of vasa previa ranges from 2 : 10,000 to 4 : 10,000 deliveries. Risk factors include a velamentous cord insertion, a low-lying placenta or placenta previa, multiple gestations, as well as bilobed or succenturiate lobe placentas. Vasa previa has been reported in as many as 50 : 10,000 pregnancies following in vitro fertilization. One systematic review reported that 83% of cases had one or more known risk factors.
Etiology and Pathophysiology
The proposed etiology of vasa previa is that there is initial central implantation of the umbilical cord vessels but with trophoblastic growth of one pole of the placenta toward the well-vascularized uterine fundus; the cord insertion becomes progressively more peripheral and eccentric until the vessels are surrounded only by fetal membranes. When these vessels traverse between the cervix and the presenting part of the fetus, it is a vasa previa.
Manifestations of Disease
Before the wide use of transvaginal ultrasound (US), the classic presentation of vasa previa included either spontaneous vessel rupture or rupture at amniotomy with vaginal bleeding causing fetal shock or death. If a vasa previa has not been diagnosed or suspected prenatally, it should be considered when vaginal bleeding occurs after membrane rupture and results in fetal heart rate abnormalities, particularly a sinusoidal pattern.
With the common use of transvaginal US, it is more often diagnosed during the middle of the second trimester. These cases may resolve over time, allowing for vaginal delivery at term.
Imaging Technique and Findings
During an anatomic survey of the fetus, a detailed assessment of the placenta and placental cord insertion should be performed. A velamentous cord insertion is characterized by splaying of the vessels at the periphery of the placenta. Once a velamentous cord insertion or an accessory placental lobe is visualized, transvaginal sonography with color Doppler should be performed to look specifically for vasa previa ( Fig. 107.1 ). Diagnosis is based on the identification of membranous fetal vessels passing across, or within 2 cm, of the internal os ( Figs. 107.2 and 107.3 ). Pulsed Doppler can be used to differentiate from maternal vessels by demonstrating a rate consistent with the fetal heart. In addition, a vasa previa can be distinguished from a funic presentation by changing the mother’s position or placing her in the Trendelenburg position, as the umbilical cord should move in the setting of a funic presentation. In a prospective trial by Sepulveda et al., velamentous cord insertion was detected using color Doppler with a sensitivity of 100%, specificity of 99.8%, positive predictive value of 83%, and negative predictive value of 100%. After suspicion of a velamentous umbilical cord, 78% of vasa previas were diagnosed in asymptomatic women prenatally. Prenatal diagnosis of vasa previa is crucial as it has been found to significantly decrease perinatal morbidity and mortality. Undiagnosed cases of vasa previa have resulted in significantly lower 5-minute Apgar scores, and increased frequencies of neonatal acidemia and demise. Surviving infants require more blood transfusions.