Placenta that implants at the lower end of the uterus. Normally, the placenta implants in the mid or upper portion of the uterus. If it implants in the lower section, the placenta may separate from the uterine wall as the uterus expands and the cervix dilates. An extreme form of this is placenta previa. On occasion a low lying placenta will appear to migrate up the uterine wall over the course of the pregnancy. This puts the placenta in a safer position. Ultrasound evaluation is essential in monitoring the position of the placenta.