Rupture of fetal vessels on placental surface.

Six placentae with subamniotic haematomata were studied and the lesions shown to be derived from ruptured varices of surface fetal veins. However, the changes in the vessel walls suggested that these fetal varices had been subjected to an increase of pressure for some time. Furthermore, the fetal veins of the affected placentae were lined by thick intimal cushions that were derived from organizing thrombi. Analysis of the clinical features of these pregnancies suggested an association of the vascular abnormalities with infants of low birthweight score, an opinion that was strengthened by an assessment of the fetal vessels in 100 placentae from normal pregnancies and 35 placentae from a `fetus-at-risk' study.