Blood cells-red, which carry oxygen, white, which fight infection, and platelets, which help with clotting-are produced in your bone marrow. They are carried throughout your body in plasma, a pale yellow mixture of water, proteins (produced primarily in your liver) and salts. There are four main types of blood, and each type can be either positive or negative. (See chart below.)

The positive or negative nature of your blood type is called your Rh factor. You may have heard about Rh factor in connection with pregnancy. About 85 percent of Americans have Rh-positive blood. If you have Rh-negative blood and get pregnant by a man with Rh-positive blood, your children will most likely have Rh-positive blood. In every pregnancy, some blood cells from the fetus may pass through the placenta and enter your bloodstream. You may react to these "foreign" invaders as if you were allergic to them, building up antibodies capable of destroying them.

This typically is not a problem for first pregnancies. But complications are possible with future pregnancies. By then, enough antibodies may have built up that when they pass through the placenta to enter the fetus's blood they begin to destroy the baby's blood cells, producing anemia and possibly resulting in the death of the baby.

To prevent complications, women who are Rh negative who have had an Rh-positive baby should receive an injection of "Rh-immunoglobulin" within 72 hours after giving birth, having an abortion or miscarrying. This safeguard prevents sensitization. In addition, expect to have the injection in the 28th week of pregnancy to prevent the few red blood cells that cross the placenta into the mother's circulation during pregnancy from starting the immunization process.