Intervillous thrombosis (IVT) is a common placental lesion characterized by thrombus formation of maternal blood in placental intervillous spaces. Found histologically in ~36-50% of placentas; most are clinically insignificant and reported as incidental findings on postpartum histological examination. On US, it appears as a well-defined, round/oval hypoechoic or echogenic lesion in the placental parenchyma — echogenicity depends on thrombus age and shows acute-subacute-chronic evolutionary change. Large (>4 cm) or multiple IVTs may reduce intervillous space perfusion, potentially associated with fetal growth restriction. AVASCULAR on Doppler — this is the most critical distinguishing feature from chorioangioma (hypervascular). T1 signal on MRI varies with thrombus age: hyperintense in subacute stage due to methemoglobin.
Age Range
18-45
Peak Age
30
Gender
Female predominant
Prevalence
Common
Intervillous thrombosis develops from thrombosis of maternal blood in intervillous spaces through several mechanisms. Three main pathogenetic mechanisms are described: (1) Villous damage — damage to the syncytiotrophoblast layer on chorionic villus surface creates an exposed thrombogenic surface, triggering maternal blood coagulation; (2) Fetal-maternal hemorrhage — leakage of fetal erythrocytes from villous damage sites into the intervillous space (fetomaternal transfusion) activates local coagulation cascade; (3) Vascular anomalies — spontaneous thrombosis develops in areas of local blood flow turbulence or stasis. As the thrombus organizes, it forms a laminar structure of fibrin, erythrocytes, and platelets — known as lines of Zahn. Echogenicity shows characteristic evolution with time: acute thrombus is hyperechoic (densely packed erythrocytes + fresh fibrin → multiple acoustic interfaces → strong echoes), subacute → erythrocyte lysis begins → decreased cellular density → mixed echogenicity, chronic → organization and liquefaction → few acoustic interfaces → hypoechoic or anechoic. MRI T1 signal depends on hemoglobin degradation stages: acute deoxyhemoglobin does not significantly affect T1 → isointense; subacute methemoglobin shortens T1 through paramagnetic effect → hyperintense (bright T1 is the most specific MR finding); chronic hemosiderin significantly shortens T2 but shows variable T1 effect. Clinical significance depends on size and number: small single IVT (<2 cm) is functionally insignificant. Large (>4 cm) or multiple (≥3) IVTs may mechanically reduce intervillous space perfusion, impairing maternal-fetal gas and nutrient exchange → IUGR. In IVTs associated with fetal-maternal hemorrhage, fetal anemia may develop — Kleihauer-Betke test confirms fetal erythrocyte passage into maternal circulation.
Well-defined, avascular lesion in placental parenchyma on Doppler — pathognomonic finding of intervillous thrombosis. Avascularity alone differentiates from chorioangioma (hypervascular). Doppler evaluation is mandatory for every intraplacental lesion.
A well-defined, round/oval lesion is seen in the placental parenchyma. Acute thrombus is hyperechoic (brighter than placental parenchyma), subacute is mixed, chronic is hypoechoic (darker) or anechoic (liquefaction). Size ranges from few mm to 5+ cm. May be multiple — number and total volume are important for functional impact assessment. Borders are usually smooth and oval/round; irregular borders may suggest alternative diagnoses.
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A well-defined, ___ (hypoechoic/echogenic/mixed) lesion measuring __ × __ mm in the placental parenchyma is consistent with intervillous thrombosis.
NO vascularity is seen within the lesion on color Doppler — avascular. Surrounding placental parenchyma vascularity is normal and normal intervillous flow may be seen around the lesion. This finding is the most critical feature distinguishing IVT from chorioangioma (hypervascular — multiple capillary channels + feeding artery). Doppler evaluation is mandatory whenever an intraplacental lesion is detected — B-mode morphology alone is insufficient for differentiation.
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The lesion is avascular on color Doppler, supporting the diagnosis of intervillous thrombosis; chorioangioma has been excluded.
On T1-weighted MRI, subacute IVT appears with high signal intensity due to methemoglobin content — bright T1 signal is the most specific MR finding for IVT. Acute IVT is isointense on T1 (deoxyhemoglobin does not significantly affect T1), chronic IVT may show low signal (hemosiderin and fibrosis). T1 hyperintense intraplacental lesion strongly suggests IVT in differential diagnosis.
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A high signal intensity lesion in the placental parenchyma on T1-weighted MRI is consistent with subacute intervillous thrombosis.
IVT shows variable signal on T2-weighted MRI: acute (deoxyhemoglobin) → low T2 signal, subacute (methemoglobin — intracellular/extracellular) → high T2 signal (especially extracellular methemoglobin), chronic (hemosiderin) → low T2 signal with rim (hypointense ring). T1 and T2 signal combination is used for thrombus aging and provides timing diagnosis.
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A lesion with variable signal intensity in the placental parenchyma is seen on T2-weighted MRI — signal pattern dependent on thrombosis age.
Lesion size (in 3 dimensions) and number are systematically evaluated. Single, small (<2 cm) IVT is clinically insignificant — no additional follow-up beyond routine obstetric care needed. Large (>4 cm) or multiple (≥3) IVTs carry fetal growth restriction risk — serial US for fetal growth monitoring and MCA PSV follow-up recommended. Total affected area can be proportioned to placental volume to indirectly estimate functional impact.
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__ IVTs are seen in the placental parenchyma with the largest measuring __ × __ mm; total affected area has been assessed.
When large or multiple IVTs are detected, fetal growth parameters (biparietal diameter, head circumference, abdominal circumference, femur length) and estimated fetal weight are evaluated. If IUGR findings (estimated fetal weight <10th percentile, abnormal umbilical artery Doppler) are present, the functional impact of IVTs becomes more significant.
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Fetal growth parameters have been evaluated — estimated fetal weight is at __ percentile; functional impact of IVTs should be considered.
Criteria
<2 cm size, single lesion, no accompanying complications
Distinct Features
Clinically insignificant, histological incidental finding, no additional follow-up needed, routine obstetric care sufficient
Criteria
>4 cm size or ≥3 lesions
Distinct Features
Increased IUGR risk, serial US fetal growth monitoring and MCA PSV monitoring recommended, umbilical artery Doppler follow-up, obstetric consultation
Criteria
Kleihauer-Betke test positive, fetal anemia findings (MCA PSV >1.5 MoM), evidence of fetomaternal transfusion
Distinct Features
High fetal anemia risk, MCA PSV monitoring mandatory, intrauterine transfusion may be needed, anti-D immunoglobulin administration in Rh incompatibility
Distinguishing Feature
Chorioangioma is HYPERVASCULAR on Doppler (multiple capillary channels + feeding artery); IVT is AVASCULAR — Doppler alone provides diagnostic differentiation
Distinguishing Feature
Abruption hematoma is retroplacental with acute painful bleeding; IVT is intraplacental within parenchyma and usually asymptomatic — location and clinical presentation difference
Distinguishing Feature
Subchorionic hematoma is crescent-shaped between chorionic membrane and uterine wall; IVT is round/oval intraplacental lesion — location (subchorionic vs intraplacental) and shape (crescent vs round) difference
Distinguishing Feature
Infarct may be wedge-shaped extending to placental edge; IVT is usually round/oval and may be anywhere in parenchyma — shape and T2 signal pattern differ (infarct: low T2, IVT: variable with age)
Urgency
routineManagement
surveillanceBiopsy
Not NeededFollow-up
6-monthSmall isolated IVT (<2 cm) is clinically insignificant and requires no specific follow-up beyond routine obstetric care — reported as an incidental finding. For large (>4 cm) or multiple (≥3) IVTs, increased obstetric surveillance is recommended: serial US for fetal growth monitoring (biometry every 2-4 weeks), MCA PSV measurement for fetal anemia screening (>1.5 MoM anemia suspicion), umbilical artery Doppler (placental vascular resistance assessment), AFI measurement. If fetal-maternal hemorrhage is suspected (IVT + fetal anemia findings), Kleihauer-Betke test is ordered from maternal blood — positive result confirms fetomaternal transfusion. Anti-D immunoglobulin is administered in Rh-negative mothers. If fetal anemia develops, intrauterine transfusion indication is evaluated. No specific medical treatment — anticoagulant therapy (heparin) has been tried in some cases but evidence level is low. Postpartum placental examination provides histological confirmation — laminar thrombus structure (lines of Zahn) is characteristic.
Most intervillous thromboses are benign and clinically insignificant — reported as incidental US findings. Large (>4 cm), multiple, or massive intervillous thrombosis may reduce functional placental surface area, being associated with fetal growth restriction, oligohydramnios, and fetal distress. Recurrent intervillous thrombosis should be investigated for thrombophilia. Size monitoring and fetal growth monitoring are recommended.