My wife is in her 23rd week of pregnancy. She has been bleeding since the 18th week

Good evening all, 

My wife is in her 23rd week of pregnancy. She has been bleeding since the 18th week. We do scan almost every two weeks and the baby is always seen doing well. We are really worried at seeing the blood. Please is this normal or what should we do about it? Doctors have confirmed placenta preavia. Kindly advise.