Vasa previa is the condition where fetal vessels (umbilical or chorionic vessels) cross the internal cervical os freely over the fetal membranes — vessels are not protected by the placenta and not supported by Wharton's jelly. Incidence is ~1/2500 pregnancies. Risk factors: velamentous cord insertion, bilobed/succenturiate placenta, IVF pregnancy, low-lying placenta, and multiple gestation. At membrane rupture (natural or amniotomy), these unprotected vessels can tear causing rapid fetal exsanguination — fetal blood volume is low (term: ~250 mL), making fatal outcome possible within minutes. Prenatal diagnosis is life-saving: undiagnosed fetal mortality is ~60%, while with prenatal diagnosis and planned cesarean it drops to <5%. Color Doppler on US demonstrates fetal vessels at membrane level over the internal os showing flow synchronous with fetal heart rate — this finding is pathognomonic.
Age Range
18-45
Peak Age
30
Gender
Female predominant
Prevalence
Fetal vessels cross the internal os through two main mechanisms: (1) Velamentous cord insertion (Type 1) — the umbilical cord inserts into fetal membranes instead of the placental edge. In normal insertion, umbilical vessels enter placental tissue directly, protected by Wharton's jelly; in velamentous insertion, vessels first run freely over membranes before reaching the placenta. This free vessel segment may cross the internal os. Wharton's jelly — a protective mucopolysaccharide gel containing hyaluronic acid, chondroitin sulfate, and type I/III collagen — is absent in this region, leaving vessels unprotected against mechanical trauma (compression, stretching, tearing). (2) Succenturiate lobe (Type 2) — connecting vessels (chorionic arteries and veins) between accessory placental lobe and main lobe run over membranes. These vessels also lack Wharton's jelly and placental tissue protection. Vessels crossing the internal os may tear at natural membrane rupture (labor onset) or iatrogenic amniotomy → fetal blood loss begins. Since fetal blood volume at term is only ~250 mL (less in preterm), even 50-100 mL loss causes hemodynamic instability and fetal exsanguination can develop within minutes. Ultrasonographic diagnosis is based on demonstrating vessels at membrane level over the internal os using color Doppler. Pulse-wave Doppler confirmation of flow synchronous with fetal heart rate (120-160 bpm) provides definitive differentiation from maternal vessels (60-100 bpm). TVUS is much more sensitive than transabdominal US as high-frequency probes positioned near the cervical region resolve thin vessels with high resolution.
Vessels showing flow synchronous with fetal heart rate at membrane level over the internal os on color Doppler — pathognomonic finding. Prenatal diagnosis reduces fetal mortality from 60% to <5% and enables safe delivery through planned cesarean.
Fetal vessels running at membrane level over the internal os are demonstrated on TVUS with color Doppler. Vessels are usually seen as 1-2 arteries and 1 vein, with diameters of 2-4 mm. Vessels course over membranes crossing the os along the cervical canal. Pulse-wave Doppler confirms flow synchronous with fetal heart rate (120-160 bpm) — this confirmation definitively differentiates from maternal vessels (cervical or uterine branches, 60-100 bpm) and establishes the diagnosis.
Report Sentence
Fetal vessels showing flow synchronous with fetal heart rate at membrane level over the internal os are demonstrated on color Doppler, consistent with vasa previa.
The umbilical cord is seen inserting into fetal membranes instead of the placental edge — velamentous cord insertion. Vessels extend over membranes from the insertion point to the placenta without Wharton's jelly protection in this membrane segment. This anatomic variant is the most important indicator of vasa previa risk and cord insertion site should be evaluated on every prenatal US. In normal insertion, the cord enters placental parenchyma directly.
Report Sentence
Velamentous insertion of the umbilical cord into membranes is seen; relationship of connecting vessels to internal os has been evaluated by Doppler for vasa previa risk.
An accessory (succenturiate) lobe separate from the main placental lobe is seen — connecting vessels between lobes run over membranes. Whether these vessels cross the internal os is systematically checked with Doppler. Succenturiate lobe is the cause of Type 2 vasa previa, and connecting vessel-os relationship must be evaluated in every succenturiate lobe diagnosis on prenatal US.
Report Sentence
Connecting vessels between succenturiate lobe and main lobe ___ (are crossing / are not crossing) the internal os — ___ (vasa previa diagnosed / vasa previa excluded).
On T2-weighted MRI sequences, vascular structures showing flow-void (signal loss) may be seen in the internal os region — representing fetal vessels. Slow venous flow may give signal and appear as hyperintense tubular structures. MRI is complementary in posterior placenta, obese patients, or cases where definitive US diagnosis cannot be made. Sagittal and coronal planes provide full evaluation of vessel course.
Report Sentence
Tubular vascular structures showing flow-void in the internal os region on MRI are consistent with vasa previa.
Cervical length and relationship of fetal vessels to the os are evaluated together on TVUS. Short cervix (<25 mm) + vasa previa combination significantly increases early membrane rupture and urgent vessel tearing risk — affects hospitalization decision and delivery timing. Distance of vessels to the os and potential compression/tearing risk during labor are assessed. Serial measurements monitor cervical length changes.
Report Sentence
Cervical length measures __ mm; delivery planning and hospitalization decision are required in combination with vasa previa.
If vaginal bleeding develops during labor, APT (Apt-Downey) test can determine whether blood is fetal or maternal. Fetal hemoglobin (HbF) is alkali resistant; maternal hemoglobin (HbA) is denatured by alkali → color change difference. Correlation of US findings with clinical test supports emergency management. Prenatal diagnosis aims to prevent this scenario — in undiagnosed vasa previa, the first sign may be fetal bradycardia + vaginal bleeding.
Report Sentence
In clinical vaginal bleeding, APT test should confirm presence of fetal blood — confirmatory for vasa previa diagnosis.
Criteria
Umbilical vessels from velamentous cord insertion cross the os
Distinct Features
Most common type (90%+), umbilical artery and vein run over membranes, cord insertion is in membranes away from placental edge
Criteria
Connecting vessels between succenturiate and main lobe cross the os
Distinct Features
Associated with bilobed/succenturiate placenta, connecting vessels may be thicker, distance between lobes and os relationship evaluated
Criteria
Vasa previa + cervical length <25 mm — high-risk combination
Distinct Features
Early hospitalization (28-30 weeks), antenatal corticosteroids mandatory, earlier cesarean may be planned (34 weeks), emergency cesarean readiness at all times
Distinguishing Feature
In previa, placental tissue directly covers internal os → maternal bleeding risk; in vasa previa, only fetal vessels cross the os without placenta → fetal bleeding risk — bleeding source (maternal vs fetal) and risk type differ
Distinguishing Feature
In succenturiate lobe, connecting vessels may not cross the os → no vasa previa; in vasa previa, vessels cross the os → planned cesarean mandatory — Doppler confirms os relationship, if not crossing then succenturiate lobe diagnosis remains
Distinguishing Feature
Cervical ectopic has gestational sac within cervix; vasa previa has no cervical pathology, only fetal vessels coursing over the os — entirely different pathologies but both involve cervical region evaluation
Urgency
emergentManagement
surgicalBiopsy
Not NeededFollow-up
specialist-referralPrenatal diagnosis of vasa previa is life-saving — undiagnosed fetal mortality is ~60%, dropping to <5% with prenatal diagnosis. Management depends on prenatal diagnosis status. If prenatally diagnosed: planned cesarean delivery is standard — usually performed at 34-37 weeks (varies by center and risk factors); delivery must be achieved before membrane rupture. Amniotomy is ABSOLUTELY contraindicated — artificial membrane rupture tears unprotected fetal vessels. Hospitalization is usually recommended at 30-34 weeks — to enable emergency cesarean at sudden membrane rupture. Antenatal corticosteroids (betamethasone/dexamethasone) are administered for fetal lung maturation — preparation for preterm delivery possibility. With short cervix (<25 mm), earlier hospitalization and cesarean are planned. Serial TVUS monitors cervical length, vessel position changes, and preterm delivery risk. Continuous fetal heart rate monitoring during delivery is mandatory — sudden fetal bradycardia suggests vessel rupture → emergency cesarean. If not prenatally diagnosed: vaginal bleeding + fetal bradycardia during labor → APT test (fetal blood detection) → emergency cesarean. Screening recommendations: when velamentous cord insertion or succenturiate lobe is detected, connecting vessel-os relationship should be evaluated with TVUS Doppler — cord insertion site should be routinely evaluated at second trimester anatomic screening.
Vasa previa leads to devastating outcomes without prenatal diagnosis — fetal vessels tear during membrane rupture, causing rapid fetal death. With prenatal diagnosis, planned cesarean (34-37 weeks) raises fetal survival to >95%. Velamentous insertion and succenturiate lobe should be screened at second-trimester US. After diagnosis, hospitalization (32-34 weeks) and antenatal steroids are administered.