What happens if there is a blood incompatibility during pregnancy?
Little harm comes from ABO incompatibility (that would have been a major problem with reproduction.) Rh incompatibility is more dangerous, though.

If a fetus is Rh+ and the mother is Rh-, when there is mixing of maternal-fetal blood (at birth, but sometimes before that), the mother will form antibodies to the blood cell surface antigen. It doesn't usually affect the first baby (not sufficient antibodies), but if a subsequent baby is Rh+, there can be major consequences, which are multiplied with repeated exposure. Those maternal antibodies - which cross the placenta - can attack the fetal red blood cells and cause destruction of many of them (called hemolytic anemia) which, if severe enough, can be fatal to the fetus. It was a not-uncommon cause of fetal demise before Rhogam (Rh immune globulin) injections.

The immune globulin shots are given to women who are Rh- with Rh+ mates at a period before birth (before blood mixing), and immediately after birth if the child is Rh+. The antibodies bind with Rh+ antigens and "mask" the antigen from the mother's immune detection, so to speak, preventing the mother from making antibodies that can be used against subsequent pregnancies.

The major risk is to the fetal blood cells, not the placenta. The placenta isn't attacked. But with anemia, the fetus struggles, for example, without enough blood, the fetus can go into congestive heart failure and die.

It's also possible to form anti-Rh antibodies from blood transfusions. If an Rh- female has had a blood transfusion prior to her first pregnancy, maternal antibody levels will be drawn earlier during the pregnancy.

What Is Rh Incompatibility?