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As far as I know the fetus is fed by the umbilical cord which basically connects his/her circulatory system with the mother's in order to supply oxygen and nutrients. But how is this possible if they (the mother and the fetus) have different incompatible blood types?

It is possible that the father has A blood type, the mother B and the child A and thus incompatible with the mother's.

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The fetus makes its own erythrocytes from a combination of the father's and mother's genome. If the mother is B and the father is A, the child will be AB as they are co-dominant. Only 'O' is recessive. This may be a too-basic question for this site... –  MCM Feb 19 '13 at 0:06
    
If the mother is BB and the father is AA. In practice, the kid could be AB, B, A, or even O, if the mother and father are both heterozygous. BO/AO. –  Jeremy Kemball Aug 14 '13 at 20:22
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1 Answer

up vote 9 down vote accepted

The blood of the fetus does not mix with the blood of the mother. Instead, the placenta provides a system where the two separate blood streams flow past each other with thin separation allowing nutrients to flow between the two streams but the not the blood cells and other large components.

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what's more, fetuses use a different set of hemoglobin genes, designed to take oxygen from the hemoglobin in mother's blood stream by diffusion through the placenta. –  shigeta Feb 19 '13 at 3:03
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