Placenta variations: Circummarginate and Circumvallate
Circumvallate: This is when the fetal membranes create an edge of double folded membranes. It creates a thickened ring that makes a smaller circle (inwards from the edge of the placenta). The exact cause is unknown but midwife Patricia Edmonds says it can be caused by abnormal implantation of the placenta and the placenta and uterine wall growing at different rates.
Circummarginate: On this placenta, the point where the membranes attach is inside the edge of the placenta. The margin is thin and flat (whereas the circumvallate placenta has a thick ridge).