Vasa Previa

01Pathophysiology

In a normal pregnancy, the umbilical vessels are protected within the umbilical cord by Wharton jelly and insert directly into the placental disc. In vasa previa, the fetal blood vessels run through the chorioamniotic membranes and cross over (or near) the internal cervical os, completely unprotected by either Wharton jelly or placental tissue. This occurs through two primary mechanisms.

Type I vasa previa arises from a velamentous cord insertion, where the umbilical cord inserts into the membranes rather than the placental disc. The vessels must then travel through the membranes before reaching the placenta, and if their path crosses the cervical os, vasa previa exists.

Type II vasa previa occurs when fetal vessels run between lobes of a succenturiate (accessory) placenta or between a bilobed placenta, crossing over the internal os in the process.

The clinical consequence is straightforward but catastrophic. When membranes rupture, whether spontaneously or during amniotomy, these unprotected fetal vessels tear open. Because fetal blood volume is very small (approximately 80 to 100 mL/kg), even a modest hemorrhage from a ruptured vessel can result in rapid fetal exsanguination and death. The bleeding is therefore fetal in origin, not maternal. This distinction is critical: the mother remains hemodynamically stable while the fetus deteriorates rapidly. Fetal mortality in undiagnosed vasa previa with ruptured membranes exceeds 50 to 60%, whereas prenatal diagnosis with planned cesarean delivery achieves survival rates above 95%.

02Classification and Clinical Manifestation

Manifestation By Scenario

The hallmark presentation on an exam vignette is the triad of: (1) rupture of membranes, (2) painless vaginal bleeding, and (3) acute fetal heart rate abnormalities with a stable mother. This triad should immediately trigger vasa previa as the leading diagnosis.

03Diagnostic Workup

Prenatal diagnosis is the most important concept here. Transvaginal ultrasound with color Doppler is both the best initial and confirmatory test. It directly visualizes fetal vessels overlying the internal os. Current guidelines recommend that when risk factors are present (velamentous insertion, succenturiate lobe, low-lying placenta, IVF pregnancy), targeted evaluation for vasa previa should be performed during the second-trimester anatomy scan. Transvaginal color Doppler has a sensitivity exceeding 90% in experienced hands.

When a patient presents acutely with vaginal bleeding at the time of membrane rupture, the Apt test can confirm that the blood is fetal in origin. This is a rapid bedside test: a sample of vaginal blood is mixed with sodium hydroxide (NaOH). Fetal hemoglobin (HbF) resists alkali denaturation and the solution stays pink, whereas adult hemoglobin (HbA) denatures and the solution turns yellow-brown. A positive Apt test (blood is fetal) in the setting of membrane rupture strongly supports the diagnosis. However, in practice, the clinical picture is often so acute that there is no time for this test, and emergent cesarean delivery should not be delayed to perform it.

Fetal heart rate monitoring is critical once bleeding begins. A sinusoidal pattern is the classic tracing associated with fetal anemia and hemorrhage. This pattern, combined with membrane rupture and painless bleeding, is a high-yield exam combination.

04Management and Treatment

Prenatal diagnosis changes everything. When vasa previa is identified before labor, the standard approach is elective cesarean delivery at 34 to 37 weeks of gestation, before the onset of labor or spontaneous membrane rupture. Many guidelines and institutions recommend 35 to 36 weeks as the optimal window, balancing prematurity risks against the danger of unplanned membrane rupture. Antenatal corticosteroids (betamethasone 12 mg IM x 2 doses, 24 hours apart) are administered if delivery is planned before 37 weeks.

Patients diagnosed prenatally are often hospitalized at 30 to 34 weeks for inpatient monitoring, given the catastrophic consequences of membrane rupture outside of a surgical setting. Pelvic rest (avoidance of intercourse and vaginal examinations) is maintained throughout.

Amniotomy is absolutely contraindicated when vasa previa is known or suspected. Artificial rupture of membranes will tear the fetal vessels and cause hemorrhage.

In the acute, undiagnosed scenario where a patient presents with membrane rupture, vaginal bleeding, and fetal heart rate abnormalities, the only management is emergent cesarean delivery. There is no role for tocolysis, expectant management, or vaginal delivery. Speed of delivery is the primary determinant of fetal survival. Neonatal resuscitation teams should be on standby, as the neonate will likely require volume resuscitation and possibly blood transfusion for hemorrhagic shock.

05Differential Diagnosis and Distractors

The most commonly confused diagnosis on exam is placenta previa. Both present with painless bleeding. The distinguishing feature is timing relative to membrane rupture: in vasa previa, bleeding starts at the moment of rupture, while placenta previa bleeds spontaneously (often provoked by cervical dilation or contractions). Additionally, in vasa previa the fetus deteriorates rapidly while the mother is stable, whereas in placenta previa the mother may show hemodynamic instability from her own blood loss.

Cord prolapse is another important distractor. Both cause acute fetal distress after membrane rupture. The difference is that cord prolapse does not produce vaginal bleeding (the cord is compressed but not torn), and the prolapsed cord can be palpated or visualized in the vagina.

06Traps and High-Yield Pearls

The most common way students miss vasa previa questions is by defaulting to placenta previa whenever they see painless vaginal bleeding in the third trimester. The critical detail that shifts the diagnosis is the temporal relationship between membrane rupture and onset of bleeding. If the vignette states "bleeding began immediately after spontaneous rupture of membranes" or "bleeding noted after amniotomy," vasa previa should be the leading diagnosis, not placenta previa.

A second trap involves the sinusoidal fetal heart rate pattern. Students may recognize this as a sign of fetal anemia but fail to connect it to fetal hemorrhage from ruptured vasa previa. When a sinusoidal tracing appears alongside membrane rupture and vaginal bleeding, the test-writer is asking you to identify fetal blood loss, not simply "fetal anemia" in the abstract.

The third high-yield concept is that prenatal diagnosis is the single most important prognostic factor. Exam questions may present a patient with known velamentous insertion or a succenturiate lobe and ask for the next best step. The answer is transvaginal ultrasound with color Doppler to evaluate for vasa previa, followed by planned cesarean delivery if confirmed. Questions testing this concept reward students who understand that the management of vasa previa is fundamentally about prevention of membrane rupture, not about treating hemorrhage after the fact.

Finally, remember that the Apt test distinguishes fetal from maternal blood. If a question provides vaginal blood that "remains pink after adding alkali," this confirms fetal hemoglobin and points toward vasa previa rather than a maternal bleeding source.