How Rh incompatibility is not much significant in first pregnancy?
In case of first pregnancy, Rh incompatibility usually isn't a problem because unless there's some sort of abnormality, the fetus's blood does not normally enter the mother's circulatory system during the course of the pregnancy. However, during delivery, the mother's and baby's blood can intermingle. If this happens, the mother's body recognizes the Rh proteins as a foreign substance and can begin producing antibodies against it. These antibodies are introduced into mother’s blood.