I am 25 weeks pregnant with complete placenta previa my doctor said it won t move and I will need a csection how likely is it that I will need a blood?

Possible. It depends on how much you bleed during pregnancy and at the time of delivery. I f you had a previous cesarean section, your risk of having placenta accreta (placenta invading into the uterine wall) is increased as well as that of blood transfusion. Talk to your doctor.