Rhesus (Rh) typing is used to find out if you have a certain protein called Rh factor on the outer layer of your red blood cells. If you don't have Rh factor in your blood, you are Rh negative. If you do have Rh factor in your blood, you are Rh-positive. Most people are Rh positive. But Rh-negative blood types can be passed down from parents to children.
Rh typing is important during pregnancy. If you are Rh negative and your baby is Rh positive, you may have an Rh incompatibility. During pregnancy, it's possible that some of your baby's blood could pass through the placenta into your body. Your body may react to the baby's blood as a foreign substance and make antibodies against it. This can cause a miscarriage, anemia, and problems in later pregnancies. The first pregnancy is often not affected by Rh incompatibility because the baby is often born before many of the antibodies develop. These risks can be lowered by a shot or shots of Rh immunoglobulin during each pregnancy, miscarriage, or other instances when you may have been exposed to Rh-positive blood.