Early in your pregnancy your blood will be tested to determine your blood type and your Rh status — that is, whether you have the Rh (Rhesus) factor, a protein that most people have on the surface of their red blood cells.
If you do have the Rh factor, as most people do, your status is Rh-positive. (About 85 percent of Caucasians are Rh-positive. If you’re Rh-negative, there’s a good chance that your blood is incompatible with your baby’s blood, which is likely to be Rh-positive. You probably won’t know this for sure until the baby is born, but in most cases you have to assume it, just to be safe.
Being Rh-incompatible isn’t likely to harm you or your baby during this pregnancy, if it’s your first. But if your baby’s blood leaks into yours (as it can at certain times during pregnancy and at birth), your immune system will start to produce antibodies against this Rh-positive blood. If that happens, you’ll become Rh-sensitized — and the next time you’re pregnant with an Rh-positive baby, those antibodies may attack your baby’s blood.
If you’re Rh-negative and you’ve been pregnant before but didn’t get this shot, another routine prenatal blood test will tell you whether you already have the antibodies that attack Rh-positive blood.