Vasa previa is a rare but serious pregnancy complication involving fetal blood vessels positioned near the cervical opening. This structural anomaly of the placenta or umbilical cord causes significant concern due to potential risks to the fetus. The condition necessitates careful monitoring and a proactive delivery plan. This article explores the nature of vasa previa and the likelihood of it resolving as the pregnancy advances.
Understanding Vasa Previa and Associated Risks
Vasa previa is defined by unprotected fetal blood vessels running through the membranes that cross the internal opening of the cervix (internal os). Normally, umbilical cord vessels are safely contained within the cord or protected by placental tissue. In vasa previa, the vessels are exposed within the amniotic membrane, leaving them vulnerable to injury.
The primary danger is the risk of vessel rupture, which can occur when the amniotic sac breaks or during labor as the cervix dilates. Since these vessels carry the baby’s blood, a tear leads to rapid and severe fetal blood loss (exsanguination). Undiagnosed vasa previa has historically been associated with a high rate of fetal death due to catastrophic hemorrhage.
The condition often occurs alongside other placental abnormalities. These include velamentous cord insertion, where the umbilical cord attaches to the membranes instead of the placenta. Another factor is a bilobed or succenturiate-lobed placenta, where blood vessels travel between separate lobes. A history of a low-lying placenta or placenta previa that resolved earlier in pregnancy can also increase the risk.
The Possibility of Apparent Resolution
Whether vasa previa resolves depends on the timing of the diagnosis. True vasa previa, where vessels cross the internal os late in pregnancy, is a fixed structural anomaly that does not spontaneously disappear. However, apparent resolution is frequently observed when the condition is identified early in the second trimester.
This phenomenon is often described as “placentation shift” or “migration,” even though the placenta does not physically move once implanted. As the pregnancy progresses, the lower uterine segment grows and stretches. This growth pulls the placental tissue and associated blood vessels upward, away from the internal cervical os.
If vessels are detected near the cervix during the routine anatomy scan (18 to 24 weeks), there is a chance they will move further away as the uterus expands. Vasa previa diagnosed in the second trimester may resolve in approximately 17% to 25% of cases. If fetal vessels remain within two centimeters of the internal os in the late second or third trimester, the condition is persistent and requires active management.
Diagnostic Procedures and Ongoing Monitoring
Vasa previa is typically diagnosed incidentally during routine prenatal ultrasound screening, as it rarely presents with maternal symptoms. The standard approach involves transvaginal ultrasound combined with color Doppler imaging. Transvaginal ultrasound provides a clear view of the relationship between the fetal vessels, the placenta, and the internal cervical os.
Color Doppler highlights blood flow, allowing clinicians to confirm that structures near the cervix are active fetal blood vessels. Diagnosis is confirmed if fetal vessels are seen lying directly over or within two centimeters of the internal os.
Following initial diagnosis, sequential monitoring scans are required throughout the second and third trimesters. These follow-up ultrasounds track vessel location to determine if the placentation shift has occurred. The frequency of these scans is tailored by the healthcare provider to ensure persistent vasa previa is confirmed and managed appropriately.
Standard Medical Management
If vasa previa is confirmed to be persistent, the standard of care shifts to proactive medical management to ensure fetal safety. The primary goal is to prevent membrane rupture and the onset of labor. This plan typically involves hospitalization in the late third trimester, often starting around 30 to 32 weeks.
Hospitalization allows for continuous monitoring and provides immediate access to an operating room if an emergency arises. Between 28 and 32 weeks, corticosteroids may be administered to accelerate the development of the baby’s lungs, preparing them for a possible early delivery.
Delivery is almost always planned as an elective Cesarean section, scheduled before the mother reaches full term and before labor begins. The recommended timing is typically between 34 and 37 weeks of gestation. This planned intervention maximizes the chances of a healthy outcome, successfully reducing the fetal mortality rate to nearly zero.