"There is an inward insertion of membranes from the edge towards the centre of the placenta (from deep implantation of the placenta into the decidua). Because of this excessive implantation, the placenta covers more than half of the fetal sac. The placenta reduces this excessive covering to the normal one-fourth by separating from the uterine wall, with the resultant back folding of the placenta and fetal membranes towards the chorionic surface."