Circumvallate placenta is a placental morphological abnormality, a subtype of placenta extrachorialis in which the fetal membranes (chorion and amnion) "double back" on the fetal side around the edge of the placenta. After delivery, a circumvallate placenta has a thick ring of membranes on its fetal surface.The fetal surface is divided into a central depressed zone surrounded by a thickened white ring which is incomplete. [Source: Wikipedia 
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