Avascular necrosis of the femoral head is still a poorly understood process, but is due essentially to an interruption in the blood supply to the bone of the femoral head. It can occur for many different reasons but is a rare complication of pregnancy.
I have been unable to find any information relating to the implications for a further pregnancy of having had hip replacements for this condition. I have never heard of a pregnant women developing avascular necrosis of any other bone in pregnancy, and I believe this would be very unlikely. Having had hip replacements, you would certainly be protected from suffering the condition in the femoral heads in the future.