intervillous thrombosis
description
in 1935 spanner published his theory that maternal blood entered the intervillous space in streams from the spiral arteries. the density of the villi within the intervillous space determines the resistance to flow from the spiral arteriole. maternal blood penetrates the intervillous space, leaves through the interlobular regions, and returns to the maternal circulation through veins beneath the placenta. the intervillous space makes up 30% to 40% of the total placental mass. intervillous thrombosis is due to the coagulation of blood within an intervillous space. either the arterial entry or venous exit of an intervillous space may be affected by thrombosis. leakage of fetal red cells into the intervillous space may also precipitate thrombosis by activating the coagulation cascade. however, the major component of an intervillous thrombus is maternal rather than fetal blood. on pathologic examination intervillous thrombi are found in 40% of placentas. they are not associated with an increase in fetal morbidity.
diagnosis
hoogland has imaged the subtle, pulsating pattern of maternal blood entering the intervillous spaces. with uterine contractions or a redistribution of spiral arterial flow, the intervillous spaces change shape or even disappear.
differential diagnosis
the differential diagnosis of an intervillous thrombosis would include a decidual septal cyst or a placental infarct. all of these lesions are sonographically indistinguishable.
sonographic features
on antenatal sonography intervillous spaces range in diameter from 0.3 to 3.0 cm.
the larger intervillous spaces have irregular borders.
with real-time imaging, slow turbulent blood flow may be appreciated within the intervillous space.
vermeulen and co-workers proved that these sonolucent areas were, in fact, intervillous spaces by comparing ultrasound images obtained in the third trimester with the anatomy of the perfused placenta after delivery.