Hi Lei,
If your Hb drops as low as 5 at times, it may be the result of some stress, especially physical stress like a cold or illness. You should try to avoid letting it get so low and transfuse before you reach that point. You can talk to your doctor about transfusing more frequently when needed if this becomes a regual issue.
You're walking a fine line with your Hb levels. However, as long as you normally are maintaining an Hb of 7-8 with only infrequent transfusions, you won't run the risk of becoming transfusion dependent. If your Hb begins to regularly drop to 5 you won't have any choice but to begin a regular transfusion program. Many intermedias who transfuse are still able to transfuse less frequently than majors but dependence does result, because the bone marrow activity will greatly reduce and the body may cease making red blood cells completely.
I was re-reading an early post of yours and wanted to address iron supplementation. It is virtually unheard of in intermedia and major to take iron because both transfusions and hyper-absorption of iron in the gut raise iron levels to dangerous levels and adding iron only adds to the problem. It is very important that you have regular serum ferritin tests to determine your range. The iron will not have a bad effect on your baby but it will continue to add to your own iron load. I think a good diet during your pregnancy might be better than taking iron. Folic acid should be continued after the birth as it helps build red blood cells.
Your doctors should be prepared for any event during the late stages of your pregnancy. Spleen size should be closely monitored along with Hb levels. If the spleen begins to rapidly enlarge, a c-section should be considered. Transfusion preparations should be made in case blood is needed during the birth. These are precautions to ensure both your health and the health of the baby, and in most cases, the most that is ever required is a transfusion at some point, but it is important to be prepared.