normally, when the infant is in the womb, the mother's blood and the baby's blood do not mix. the nutrients required by the fetus are taken up and given to him or her by the placenta while the mother's blood flows alongside it. the placenta and baby both contain the baby's blood, which is divided from the mother's blood by a membrane.
your baby's blood could become sensitive if you have RH- blood and your infant has RH+ blood and the two mingle. your body can start making antibodies against the RH+ blood. these antibodies have the potential to start destroying the red blood cells of an unborn child if they get there. RH disease is what this is. it is uncommon for it to impact a newborn, but if the mother develops sensitivity and manufactures antibodies to RH+ blood, it could have negative effects on subsequent births. although RH disease is a highly dangerous condition, 90% of infants with RH disease survive.