Blood transfusion is indicated in the following condition with sickle cell anaemia
Indications of Blood transfusion in Sickle Cell Anemia

Frequent sickling episodes
Twin pregnancy
Poor obstetrical outcome
Severe anaemia

Blood transfusion replaces the abnormal cells with normal red cells
The objective of transfusion is to keep the hematocrit value above 25% and concentration of Hb–Sunder 50% 
Prophylactic transfusions during pregnancy remain controversial
Frequent sickling episodes
Twin pregnancy
Poor obstetrical outcome
All of the above