Vasa previa is a rare but serious complication of pregnancy that involves a life-threatening risk to the unborn baby. This condition occurs when fetal blood vessels, which are normally protected within the umbilical cord or placental tissue, run through the membranes directly over the opening of the cervix. If these unprotected vessels tear, usually when the amniotic sac ruptures, the baby can experience rapid and catastrophic blood loss. Antenatal diagnosis of this condition is paramount, as the survival rate for babies diagnosed before labor is over 95%, compared to a fetal mortality rate of at least 60% if the condition is only discovered during labor.
Understanding Vasa Previa
Vasa previa is defined by the presence of fetal blood vessels coursing within the amniotic membranes and located within two centimeters of the internal cervical os. These vessels are not cushioned by the protective Wharton’s jelly or placental tissue, leaving them highly vulnerable to rupture.
The condition commonly arises in pregnancies involving certain risk factors, such as a velamentous cord insertion (where the umbilical cord inserts into the membranes instead of the placenta). Other risk factors include a bilobed or succenturiate-lobed placenta, multiple gestations, and pregnancies resulting from in vitro fertilization (IVF).
The primary danger is that the vessels will be compressed or torn as the cervix dilates or when the membranes break. Since these vessels carry the baby’s blood, rupture leads to immediate fetal hemorrhage, or exsanguination, which can be fatal within minutes. Management is centered on preventing the membranes from rupturing before a controlled delivery can take place.
The Possibility of Spontaneous Resolution
A frequent question following an early diagnosis is whether vasa previa can resolve itself as the pregnancy progresses. Apparent resolution is possible, though it is considered a positional shift rather than a true biological resolution. Studies show that a significant proportion of cases diagnosed in the second trimester (20% to 40%) have been observed to resolve.
This apparent movement, often termed “pseudo-resolution” or “migration,” is caused by the ongoing development and stretching of the lower uterine segment, particularly after 28 weeks of gestation. As the uterus expands, the relationship between the fetal vessels and the internal cervical os changes, pulling the vessels away from the danger zone.
The likelihood of this positional shift is greater when the initial diagnosis is made earlier (before 26 weeks) and when the vessels do not completely cover the cervical opening. If the vessels are found to be more than two centimeters away from the cervical os later in pregnancy, the vasa previa is considered resolved and the high-risk management protocol may be discontinued.
Even if the vessels appear to have migrated away, serial monitoring remains necessary to confirm the sustained distance from the cervix. Due to the high risk associated with a missed or recurring diagnosis, medical professionals maintain a cautious approach. A definitive diagnosis of persistent vasa previa is typically not made until the late third trimester.
Monitoring and Diagnostic Confirmation
Once vasa previa is suspected, especially in women with identified risk factors, a standardized approach is used for diagnostic confirmation and ongoing surveillance. The definitive tool is a transvaginal ultrasound combined with color Doppler mapping.
The transvaginal approach provides a clear view of the relationship between the fetal vessels and the internal cervical os. Color Doppler confirms blood flow within the structures, and Pulsed-wave Doppler ensures the flow corresponds to the fetal heart rate, confirming the vessels belong to the baby.
Serial ultrasound evaluations are scheduled throughout the third trimester to track vessel position and monitor for migration. If diagnosed in the second trimester, follow-up scans are typically performed around 28 to 32 weeks gestation, and then every few weeks thereafter. This monitoring confirms whether the vessels have moved far enough away from the cervix to be considered resolved, or if the diagnosis persists.
Clinical Management When Resolution Does Not Occur
When serial monitoring confirms persistent vasa previa (vessels remaining within the two-centimeter threshold), a proactive management plan is implemented to maximize fetal safety. The primary goal is to keep the baby in utero as long as possible while avoiding the onset of labor or membrane rupture.
Antenatal corticosteroids are usually administered between 28 and 32 weeks of gestation to promote lung maturation, anticipating a necessary preterm delivery. Scheduled hospitalization is often considered, typically beginning around 30 to 34 weeks gestation, to ensure immediate access to an operating room should an emergency arise.
The standard of care involves a planned Cesarean delivery scheduled before the onset of labor and before the membranes rupture. For a singleton pregnancy, this delivery is commonly timed between 34 and 37 weeks gestation.
The timing balances the risks of prematurity against the risk of catastrophic hemorrhage if the pregnancy continues too long. Ensuring a controlled, pre-labor Cesarean effectively mitigates the risk of vessel rupture, leading to the near-perfect survival rate associated with antenatally diagnosed vasa previa.