In the case of pregnancy, the danger lies in a sensitized rh- mother carrying an rh fetus. how does this harm the fetus?

The mother's antibodies can destroy the red blood cells of the fetus.

Explanation:

If a woman with an Rh negative (Rh-) blood group has a child with an Rh positive (Rh +) male, there is a possibility that the child is also Rh +. Although this has no consequences throughout pregnancy itself, as the mother's and fetus's blood does not come into direct contact during pregnancy, it can have serious consequences for subsequent pregnancies.

In fact, rupture of the placenta at the time of childbirth may allow some red blood cells from the fetus to pass into the maternal circulation and cause sensitization, meaning the woman's body will detect them and begin to make anti-human antibodies. Rh that will persist indefinitely in the blood. If a woman regenerates another Rh + child, the anti-Rh antibodies present in her blood will pass easily into the fetus's blood through the placenta and attack and destroy the fetus' red blood cells, which can lead to severe fetal anemia and serious repercussions that sometimes lead to intrauterine death.