Placental abruption is a situation where the placenta separates away from the uterine wall. When this happens the baby does not receive any oxygen whatsoever from the placenta and the umbilical cord.
It is a life-threatening emergency.
Placental abruptions are usually diagnosed because mom will be in extreme pain during labor and the fetal heart rate monitor will show that the baby's heart rate is in decelerations and not recovering. This is a medical emergency and requires immediate delivery either by cesarean section or if the baby is almost out of the birth canal by vaginal delivery.
In either case, a placental abruption is a medical emergency.