INTERVILLOUS THROMBUS (IVT)

IVT is a discrete focus of coagulated blood in the intervillous space.

ETIOLOGY

Leakage of fetal red blood cells into the intervillous space results from fetal capillary vasodilatation or focal ischemia and rupture of the thin trophoblasts covering the fetal villous capillaries. A coagulation cascade results as the fetal blood is recognized as as foreign and antigenic by the maternal blood (1). The villi at the periphery may be compressed and / or infarcted. No decidual border is present (this differentiates IVT from a decidual septal cyst).

PREVALENCE

Ranges from 20-50% of full term, uncomplicated pregnancies (1).

ASSOCIATIONS

Rare (erythroblastosis fetalis or pre-eclampsia).
Elevated maternal serum AFP (2).

ULTRASOUND

• Hypoechoic area / areas in the placenta.
• Usually £3 cm (hypoechoic or mixed hypo-isoechoic lesions larger than 4-5 cm is less likely to represent IVT and may represent subchorionic fibrin deposition, intraplacental hematoma or chorioangioma).

REFERENCES

1. Fox H. Pathology of the placenta. In: Bennington JL (ed): Major problems in pathology. Philadelphia, Saunders, 1978 pp 1-37.
2. Fleischer AC, Kurtz AB, Wapner AJ et.al. Elevated alpha-fetoprotein and a normal fetal sonogram: Association with placental abnormalities. AJR 1988;150:881-883.