Just as there are different major blood groups, such as type A and type B, there also is an Rh factor. Rhesus (Rh) factor is an inherited protein found on the surface of red blood cells. The Rh factor is inherited—passed down through parents’ genes to their children. If the mother is Rh negative and the father is Rh positive, the fetus can inherit the Rh gene from the father and could be either Rh positive or Rh negative. If your blood has the protein, you’re Rh positive. If your blood lacks the protein, you’re Rh negative.
Rh positive is the most common blood type. Having an Rh negative blood type is not an illness and usually does not affect your health. However, it can affect your pregnancy. Your pregnancy needs special care if you’re Rh negative and your baby’s father is Rh positive.
The Rh factor can cause problems if you are Rh negative and your fetus is Rh positive. This is called Rh incompatibility. These problems usually do not occur in a first pregnancy, but they can occur in a later pregnancy. When an Rh-negative mother’s blood comes into contact with blood from her Rh-positive fetus, it causes the Rh-negative mother to make antibodies against the Rh factor. These antibodies attack the Rh factor as if it were a harmful substance. A person with Rh-negative blood who makes Rh antibodies is called “Rh sensitized.”During pregnancy, the woman and fetus do not share blood systems. However, a small amount of blood from the fetus can cross the placenta into the woman’s system. This sometimes may happen during pregnancy, labor, and birth. This sometimes may happen during pregnancy, labor, and birth. It also can occur if an Rh-negative woman has had any of the following during pregnancy:
- Ectopic pregnancy — when a fertilized eggs implants somewhere outside the uterus, usually in a fallopian tube
- Molar pregnancy — a noncancerous (benign) tumor that develops in the uterus
- Amniocentesis — a prenatal test in which a sample of the fluid that surrounds and protects a baby in the uterus (amniotic fluid) is removed for testing or treatment
- Chorionic villus sampling — a prenatal test in which a sample of the wispy projections that make up most of the placenta (chorionic villi) is removed for testing
- Bleeding during pregnancy
- Blunt trauma to the abdomen during pregnancy
- Rotation of a baby in a breech position — such as buttocks first — before labor
- Fetal blood sampling
Problems during pregnancy can occur when Rh antibodies from an Rh-sensitized woman cross the placenta and attack the blood of an Rh-positive fetus. The Rh antibodies destroy some of the fetal red blood cells. This causes hemolytic anemia, where red blood cells are destroyed faster than the body can replace them.
Red blood cells carry oxygen to all parts of the body. Without enough red blood cells, the fetus will not get enough oxygen. Hemolytic anemia can lead to serious illness. Severe hemolytic anemia may even be fatal to the fetus.
A blood test, called an antibody screen, can show if you have developed antibodies to Rh-positive blood and how many antibodies have been made. If you are Rh negative and there is a possibility that your baby is Rh positive, your health care provider may request this test during your first trimester and again during week 28 of pregnancy.
If you are Rh negative, you will be given a shot of Rh immunoglobulin (RhIg). RhIg is made from donated blood. When given to a nonsensitized Rh-negative person, it targets any Rh-positive cells in the bloodstream and prevents the production of Rh antibodies. When given to an Rh-negative woman who has not yet made antibodies against the Rh factor, RhIg can prevent fetal hemolytic anemia in a later pregnancy.
If you are Rh sensitized, you will be monitored during pregnancy to check the condition of your fetus. If tests show that your baby has severe anemia, it may be necessary to deliver your baby early (before 37 weeks of pregnancy) or give a blood transfusion while your baby is still in your uterus (through the umbilical cord). If the anemia is mild, your baby may be delivered at the normal time. After delivery, your baby may need a transfusion to replace the blood cells.