What Causes Bladder Enlargement After Having Normal Delivery?

my sister in law has gone through a normal delivery case on 01.12.2011.Mother and child were both fine.But later on it was diagnosed that mother is not able to pass urine.We have consulted the gyno then she has put the patient on dialysis,in that way urine is getting passed.But after removal the urine is not coming.As per the doctor the bladder has got expanded and it will take time to get normalized.
Please advice .