I received my blood test results earlier last week and learned that I have O- blood. I've always wondered what my blood type is, and before now didn't realize the importance of finding it out while being pregnant.
Because I am RH-, I must have a Rhogam (RH) shot at 28 weeks, and possibly again within 72 hours of birth, in the event our baby is born with RH+ blood (which is very likely).
According to babycenter.com, "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. Fortunately, you can avoid becoming Rh-sensitized by getting an injection of a drug called Rh immune globulin whenever there's a chance that your blood has been exposed to your baby's blood"
We're not worried - from what the nurse told me, this is extremely common. I'm actually fascinated by the entire thing, and the fact that I am a universal blood donor (but can only receive O- blood in the event of a transfusion).