There are 4 major blood types A, B, AB, or O. Blood types are determined by the types of antigens on the blood cells. Antigens are proteins on the surfaces of blood cells that can cause a response from the immune system. Type A blood has only A antigens, type B has only B antigens, type AB has both A and B antigens, and types O has neither A nor B antigens. There are other antigens that can make blood types even more specific: one of the most common is the Rh factor.
As part of your prenatal care, you will have blood tests to find out your blood type. If your blood lacks the Rh antigen, it is called Rh negative. If it has the antigen, it is called Rh positive. More than 85% of people in the world are Rh positive. When the mother is Rh negative and the father is Rh positive, the fetus can inherit the Rh factor from the father, which makes the fetus Rh positive, too. Problems can arise when the fetus’s blood has the Rh factor and the mother’s does not.
If a small amount of the fetus’s blood mixes with the mother’s blood, which often happens, the mother’s body may respond as if it were allergic to the fetus by making antibodies to the Rh antigens in the fetus’s blood. This means the mother has become sensitized. Her antibodies can then cross the placenta and attack the fetus’s blood, breaking down its red blood cells and causing anemia. This can lead to serious illness or even death in the fetus or newborn.
The first pregnancy poses little risk because it takes time for the mother’s antibodies to build up after being exposed to the antigens. But once they are formed, antibodies do not go away. The best course is to keep the mother from being sensitized and forming antibodies in the first place. If you are Rh negative and blood tests show that you have not become sensitized, your doctor will prescribe Rh immune globulin (RhIg) injections. This blood product prevents the mother from forming antibodies. It is usually injected right after the birth of your first baby to prevent harm to your next one. It may also be given during pregnancy, usually at about 28 weeks of pregnancy.
If you have already become sensitized, your fetus is at risk. As your pregnancy progresses, your doctor will check the levels of antibodies in your blood. If they become high, special tests may be done to check your fetus’s health.
If the fetus is anemic, it will need a blood transfusion. After 18 weeks of pregnancy, these transfusions can be given while the fetus is still in the uterus. If the fetus is old enough, early delivery may be an option. The baby can be treated in a special-care nursery. If you have become sensitized, other babies you have after your first may be at risk. They will to be monitored carefully throughout pregnancy.