Can blood cross the placenta? (Between a baby and a mom) is that why there are the standard injections for RH incompatibility?
I just read this just wanted to verify if it was true
Yes, blood can cross the placenta... if not there would be no need of the standard injections for RH incompatibility! The danger is NOT of the mother's blood attacking the baby's at the birth (when the baby is already formed and ready to go) but while in the womb. The mother's antibodies can attack the growing fetus and prevent it's developing mature red blood cells, thus causing severe anemia and failure to develop, and ultimately death if the mother's immune response is strong enough.
If the mother has Rh positive blood (any positive) then there will be no risk of Rh incompatibility, no matter what the father's type. There are other minor blood groups however which can cause a mismatch, but they're more rare.
Just a quick note, because I don't have time to address the whole set of questions here. An infant, by definition, means post-natal (already born). You don't have any infants in the womb. They are embryos/fetuses.