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It has nothing to do with ABO blood group
. If mother is Rh negative
and father is Rh positive, the foetus will be Rh positive and mother blood may start forming antibodies
against fetus. As the process is very slow, it shall not affect the first pregnancy
but the subsequent pregnancies become risky for fetus. To prevent this in future, some injections at first childbirth are protective against the future pregnancies.
Hope I could satisfy your query.