Why is the outcome different for the rh+ baby compared to second rh+ baby?

The risk of maternal antibodies crossing the placenta and destroying the fetal red blood cells increases with each pregnancy. So, the outcome of the first RH+ baby is different from the second one.

EXPLANATION:

RH+ is a specific antigen present on the RBCs. A person that inherits these antigens has blood group RH+ and the ones who lack them have RH- blood group. Issues occur when the woman is RH- and the fetus is RH+ is the antibodies that cross the placenta and affect the baby's RBCs negatively.

This incompatibility occurs when father is RH+ and mother is RH-. The first-time pregnancy doesn't create much of the incompatibility. But if the fetus is RH+ the second time, results in life-threatening problems. During first time pregnancy, the RH+ antibodies are not present in the mother's blood.

But during the birth when placenta ruptures, the bloodstream enters into the mother's body and it starts stimulating the antigens of RH+ blood. Now, during the second pregnancy when the antibodies transfer from the mother to the placenta, RH+ antigens react with the fetal blood cells and destroys them.