Placenta lakes. This are basically ultrasound findings and means like bubbles of blood , dilated looped of vessels. When you look an a MAP and see lakes, the image if this vessels appear the same on ultrasound. It can point sometimes towards a possible abnormal placentation, called accreta, when placenta attaches really hard to the uterus.
Usually benign. Placental venous lakes are commonly seen in prenatal ultrasound and have no clinical consequence whatsoever. Rarely, in the context of prior uterine surgery and previa, they are associated with morbidly adherent placenta called accreta. That is a potentially life-threatening condition with rapidly increasing frequency due to the c-section epidemic.