Hi, thank you for asking healthcare magic
I understand your concerns
It is not possible for both parents with
blood group A+ to give birth to a baby whose blood group is B+. If your blood group is A, it means the possible combinations you have can either be AA or AO. During the production of sperm cell or an agg, only a single allele (ie A or O) would be retained and during fertilisation, the egg or sperm will supply either A or O.
If we consider both parents to be AA, it means that all their off springs would be AA (group A)
If one of the parents is AA and the other AO, then their off springs would either be AA or AO (all of them still group A)
If both parents are AO, then their off springs could be AA, AO or OO (i.e group A or O).
From this explanation, the off springs of parents whose blood group is A+ would either be of blood group A+ or O+.
The blood group does not change with growth. It remains the same from fertilization.
I hope I have answered your query
I wish you well