A patient is 8 months pregnant. She calls her her obstetrician because of vaginal bleeding. It is "spotting" rather than heavy bleeding and there is no pain. An ultrasound at the hospital shows that the placenta is formed across the internal os of the cervix. The patient is prescribed bed rest for the remainder of the pregnancy. What name is given to this condition? Why is it considered extremely serious? Why was she ordered to have complete bed rest?