marginal placenta previa, placenta previa in which the placenta is implanted in the lower uterine segment, with its margin within 2 cm of the internal os of the uterine cervix. During labor as the cervix dilates, bleeding may occur from the separation of the edge of the placenta from the uterus beneath it. Bleeding may be so scant as to pose no clinical problem. In some cases, frank severe hemorrhage may occur, but the pressure of the presenting part of the baby is often sufficient to act as a tamponade, arresting the hemorrhage. Diagnosis of marginal placenta previa may be suggested by the apparent location of the placenta on ultrasonic visualization. Cesarean section is not usually necessary. See also placenta previa.
