Vasa previa is a rare but serious complication in pregnancy where fetal blood vessels travel unprotected across the membranes near the cervical opening (internal os). Occurring in approximately 1 in 2,500 to 5,000 pregnancies, this condition poses a substantial risk to the unborn baby. The vessels are vulnerable to rupture because they lack the cushioning of the umbilical cord’s protective jelly or placental tissue. Prenatal diagnosis drastically improves the outcome.
Defining the Condition and Anatomical Risk
Vasa previa is defined by fetal vessels running within the amniotic and chorionic membranes, crossing the entrance to the birth canal. Typically, umbilical cord vessels are safely housed within the cord, protected by Wharton’s jelly, until they insert into the placenta. With vasa previa, these vessels are exposed as they pass over the internal cervical os.
This anatomical abnormality creates a hazard because the exposed vessels are fragile and can easily tear when the fetal membranes rupture during labor. Since these vessels contain the baby’s blood, their rupture leads to rapid fetal hemorrhage, or exsanguination. Undiagnosed vasa previa has a fetal mortality rate often exceeding 50%, which drops to less than 10% when detected before birth.
The condition is categorized into two types based on the underlying placental or cord anomaly. Type 1 is associated with a velamentous cord insertion, where the umbilical cord connects to the membranes rather than directly into the placenta tissue. The unprotected vessels then travel through the membranes to reach the main placenta.
Type 2 vasa previa occurs when the placenta is bilobed (having two main sections) or contains a succenturiate lobe (a smaller accessory lobe). In this type, unprotected vessels run through the membranes to connect these separate lobes, often passing directly over the cervix. Both types create a path of unsupported vessels susceptible to tearing.
Identifying the Risk Factors
Certain conditions significantly increase the likelihood of vasa previa, making targeted screening necessary. A history of a low-lying placenta or placenta previa, especially if resolved after the second trimester, is a common factor. The placenta’s initial position may contribute to the abnormal placement of the vessels.
Anomalies in the placenta’s structure or cord attachment are also indicators. These include velamentous cord insertion (Type 1) and the presence of a bilobed or succenturiate placenta (Type 2). These structural differences create the setting for unprotected vessels near the cervix.
Pregnancies resulting from assisted reproductive technology, such as in vitro fertilization (IVF), have an elevated risk compared to naturally conceived pregnancies. While the exact reason is not fully understood, it may relate to the implantation process. Multiple gestations also represent an increased risk due to the greater likelihood of placental abnormalities.
Prenatal Detection and Diagnosis
Early prenatal diagnosis is the most effective intervention for ensuring a positive outcome. The condition typically lacks reliable symptoms before labor begins, which is often too late to prevent fetal blood loss. Diagnosis is frequently achieved during the routine second-trimester anatomy ultrasound scan (18 to 22 weeks).
If a risk factor is identified during the initial scan (e.g., a low-lying placenta or abnormal cord insertion), a targeted examination is recommended. The standard diagnostic technique involves using a transvaginal ultrasound (TVUS) combined with color Doppler imaging. The TVUS provides a clear view of the lower uterine segment and the cervical opening.
Color Doppler highlights and confirms blood flow within any vessels seen crossing the internal cervical os. This technology allows the clinician to differentiate fetal vessels from other structures. Once identified, about 20% of cases diagnosed in the second trimester may resolve by the third trimester, requiring follow-up scans to monitor the vessels’ position.
Management Protocol and Safe Delivery
Once vasa previa is confirmed, the primary goal is to maintain the pregnancy as long as safely possible while preventing membrane rupture and the onset of labor. This requires a monitored approach throughout the third trimester. Antenatal corticosteroids are administered between 28 and 32 weeks to help mature the baby’s lungs in case early delivery is necessary.
Prophylactic hospitalization, usually considered between 30 and 34 weeks, is decided on an individualized basis. Factors like the patient’s distance from the hospital, uterine contractions, or a history of preterm birth are taken into account. The goal of hospitalization is to ensure immediate access to an operating room if spontaneous rupture of membranes or bleeding occurs.
A planned Cesarean section (C-section) is mandatory to bypass the risk of vessel rupture during vaginal delivery. The timing balances the risks of prematurity against the risk of the vessels tearing before the scheduled date. Based on current data, a planned C-section is often scheduled between 34 and 37 weeks.
Delivery should occur in a facility equipped to handle immediate complications, including providing a neonatal blood transfusion. During the C-section, the surgical team must be mindful of the vessel location to avoid accidentally cutting them when opening the uterus or membranes. This strategy results in a survival rate of over 95% for the baby.