Vasa previa is a rare but severe complication of pregnancy that occurs when fetal blood vessels run unprotected across the entrance to the birth canal. This anatomical abnormality can lead to catastrophic outcomes for the baby if not identified before labor begins. These misplaced vessels are vulnerable to damage, which can result in rapid and life-threatening blood loss for the fetus. Understanding the mechanism of vasa previa and the importance of prenatal detection is essential for ensuring a safe delivery and a positive outcome.
The Anatomy of Vasa Previa
Vasa previa is an anatomical abnormality where fetal blood vessels, connecting the umbilical cord and the placenta, traverse the membranes directly over or very close to the internal cervical os (the opening of the uterus). These vessels are unprotected because they are not encased in Wharton’s jelly, the thick, gelatinous substance that normally shields the umbilical cord vessels. Instead, they run exposed within the thin membranes of the amniotic sac, making them susceptible to rupture or compression. This exposure often stems from an abnormal cord insertion, most commonly a velamentous cord insertion, where the umbilical cord connects to the membranes instead of the placental mass itself. The condition can also be associated with placental variations, such as a bilobed or a succenturiate-lobed placenta, placing the vessels directly in the path of the descending baby.
The Severe Fetal Risks Involved
The danger of vasa previa arises when the fetal membranes rupture, either spontaneously or during medical intervention. Because the vessels are unsupported and lie directly over the cervix, they are likely to tear when the membranes break or when the cervix begins to dilate during labor. A tear in these exposed vessels causes immediate and rapid fetal hemorrhage, meaning the baby loses a massive amount of its own blood. The fetus has a relatively small total blood volume, so even a small amount of blood loss can be devastating, leading to exsanguination. The fetal mortality rate for undiagnosed vasa previa is reported to be as high as 50% to 95%. Even if the vessels do not rupture, they can become compressed between the baby’s presenting part and the birth canal, leading to fetal oxygen deprivation. This compression causes severe changes in the fetal heart rate, which is a sign of acute distress.
Detection Through Prenatal Screening
Prenatal diagnosis before labor or membrane rupture is essential to prevent the high mortality rate associated with vasa previa. Screening is often done during the routine second-trimester anatomy ultrasound, typically performed around 18 to 26 weeks of gestation. During this examination, the physician looks for risk factors like a velamentous cord insertion, a low-lying placenta, or a multilobed placenta, which warrant further investigation. If risk factors are noted, a targeted transvaginal ultrasound is performed to get a clearer view of the internal cervical os. This ultrasound is combined with color Doppler technology, which uses sound waves to visualize and map the flow of blood. Color Doppler confirms that the structures overlying the cervix are fetal blood vessels. A confirmed prenatal diagnosis significantly improves the baby’s survival rate to over 95% because it allows for a controlled, planned delivery.
Management and Delivery Protocols
Once vasa previa is diagnosed, the management plan focuses on preventing the vessels from rupturing before a safe delivery can be performed. This plan involves increased antepartum surveillance, which may include frequent non-stress tests to monitor the baby’s well-being and detect signs of vessel compression. Some patients are admitted to the hospital for continuous monitoring, often starting between 30 and 34 weeks of gestation, particularly if they have other risk factors or symptoms. To prepare for preterm birth, antenatal corticosteroids are administered, usually between 28 and 32 weeks, to accelerate the maturation of the baby’s lungs. The definitive treatment for vasa previa is a planned delivery via Cesarean section (C-section) that is scheduled before the onset of labor and before the membranes spontaneously rupture. This planned C-section is commonly scheduled between 34 and 37 weeks of gestation, balancing the risks of prematurity against the danger of vessel rupture, and ensures the baby is delivered in a controlled environment where immediate neonatal care is available.