http://familydoctor.org/familydoctor/en/pregnancy-newborns/fetal-health/rh-incompatibility.printerview.html
American Academy of Family Physicians
If you're not sure what your Rh factor is and think you're pregnant, it's important to start regular prenatal care as soon as possible — including blood-type testing. Your Rh factor may play a role in your baby's health, so it's important to know this information early in your pregnancy.