Mrs. Phang, eight months pregnant with her first child, makes an appointment wit

Mrs. Phang, eight months pregnant with her first child, makes an appointment with her obstetrician to investigate the cause of vaginal bleeding. Because it is actually more "spotting" than frank bleeding, and painless, she is becoming worried. Ultrasound visualization of her abdomen shows that the placenta is formed across the internal os of the cervix, and she is ordered to her bed for the remainder of the pregnancy. What name is given to Mrs. Phang's condition? Why is it an extremely serious condition? Why was she ordered to have complete bed rest for the remainder of her pregnancy?