A placenta characterized by fetal membranes that are folded along all or part of the disc perimeter. the amnion may or may not be included in the folded tissue.

Circumvallate placenta is a rare condition affecting about 1-2% of pregnancies, in which the amnion and chorion fetal membranes essentially "double back" on the fetal side around the edges of the placenta. After delivery, a circumvallate placenta has a thick ring of membranes on its fetal surface...