If an woman is pregnant carrying an fetus, the mother might obtain some RBCs from the fetus during the birth process or during a miscarriage.

Rh- woman,

carrying Rh + fetus.

Explanation:

Rhesus factor is a  protein on the red blood cells of pregnant woman, whose incompatibility  with the fetus Rhesus group may cause serious complication to the fetus.

Generally an individual said to be Rhesus positive Rh+,if the protein for Rhesus factor is located on the blood cells.While individual with no Rhesus factor on the blood cells is said to be Rhesus negative.(Rh-).

Thus if a Rh-  pregnant mother is caring a Rh+ child, no mixture of blood occurs since the placenta separated the two blood sources from each other.

Assuming that during birth process the placenta peels off, and some Rh+ blood of the fetus gets to the mother, then the mother  immune system will develop antibodies against the fetus Rh+ blood.

However, since the child had already been born, the antibodies will  have  no significant effect on the baby. Assuming the woman gets pregnant again with Rh+ fetus , the mother's immune systems will recognise the baby as an antigen/invader, and will therefore attack the fetus.This may be marked with  bleeding, and if proper care is not employed, it may lead to death of the child.

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