Rh incompatibility

To understand Rh incompatibility, it's first important to know something about the Rh factor of your blood. Your Rh factor is an inherited trait, as are the color of your eyes or your height. Most people have a particular protein called D-antigen on the surface of their red blood cells and, therefore, have Rh-positive (Rh+) blood. However, about 15 percent of Caucasians and a smaller fraction of people in other racial groups don't have this protein. Instead, they have Rh-negative (Rh-) blood. Whether your blood is Rh+ or Rh- does not, in itself, affect your health.

Rh incompatibility develops when Rh-positive blood cells are introduced to an Rh-negative person. This can happen when a person with Rh-negative blood gets a blood transfusion from someone with Rh-positive blood or when a pregnant woman with Rh-negative blood has a fetus with Rh-positive blood. Usually the fetal and maternal blood don't mix, but even a small amount of Rh-positive fetal blood can cross the placenta (the structure that develops in the womb to nourish the growing fetus and help it discard waste) and cause the mother to become sensitized. This means the Rh-negative mother's body produces antibodies to fight the Rh-positive fetal blood as if it were a harmful substance.

Knowing the Rh factor for both the mother and father can help determine the risk of Rh incompatibility. If the mother and father are Rh-negative or the mother is Rh-positive and the father is Rh-negative, there is no risk of Rh incompatibility. An Rh-negative mother and Rh-positive father, however, can conceive a baby who inherits the father's Rh-positive blood type.

Rh incompatibility is sometimes referred to as Rh isoimmunization, Rh alloimmunization or Rh sensitization.


Causes/associated factors
The first experience with Rh incompatibility, either through transfusion or pregnancy, usually has no consequences. This is because your body's initial immune response has a sensitizing effect. Again, this means if you're Rh-negative but your baby is Rh-positive, your body will begin to produce antibodies to fight the baby's Rh-positive blood. Once the antibodies form, they remain in your blood forever.
This first encounter, however, does make you more susceptible to incompatibility problems in the future. Now that your body has produced the antibodies, you'll have a faster and stronger reaction against subsequent exposures to Rh-positive blood. If you're Rh-negative and receive a transfusion of Rh-positive blood, for example, these antibodies will form and destroy the red blood cells in the donor blood.

If there is any mixing of maternal and fetal blood during the first pregnancy of an Rh-negative mother with an Rh-positive baby, the baby is often born before the mother makes enough antibodies to affect the baby. Rarely, enough antibodies are made to begin causing problems during the last three months of the pregnancy. In these rare first pregnancy cases and for all subsequent pregnancies, the mother's antibodies may cross the placenta and attempt to destroy the baby's red blood cells, causing anemia (lack of red blood cells to carry oxygen in the blood). This condition is known as hemolytic disease of the newborn. With careful monitoring and treatment, more than 80 percent of infants survive pregnancies with a high risk for Rh incompatibility.

Risk factors for developing Rh incompatibility during pregnancy include:

being an Rh-negative woman with an Rh-positive baby
undergoing invasive procedures associated with potential damage to the fetus, such as amniocentesis (withdrawal of amniotic fluid, the fluid that surrounds and protects the fetus in the womb, for examination) and chorionic villus sampling (a prenatal test used to detect certain genetic defects early in a pregnancy)
abnormalities with the placenta (a structure in the womb that develops during pregnancy to nourish the fetus and help it discard waste) that could lead to hemorrhage, such as an abruptio placenta (early separation of the placenta from the uterus), placenta previa (when the placenta covers the cervix) and manual removal of the placenta
maternal trauma, especially to the abdomen, or external rotation or turning of the baby
If you're an Rh-negative woman, you may have been exposed to Rh-positive fetal blood -- and thus, already have the antibodies -- if you:

have a history of fetal death or hydrops (generalized fetal swelling that may occur in conjunction with hemolytic disease)
had a miscarriage or elective abortion
had an ectopic pregnancy (in which the fertilized egg implants itself outside the uterus)
had a cesarean delivery (through an incision in your abdomen)

Signs/symptoms
Rh incompatibility may cause:
newborn jaundice (yellowing of the skin and whites of the eyes)
fetal or newborn anemia (a condition marked by a decreased number of red blood cells or hemoglobin); in severe cases, the baby may develop hydrops, a condition involving generalized fetal swelling with enlargement and swelling of the fetal liver and spleen, as well as circulatory disorders
fetal distress

Diagnosis
A simple blood test confirms blood type and the presence or absence of the Rh factor. To ensure compatibility before receiving a blood transfusion or during pregnancy, your blood will also be tested for the presence of antibodies. If you've had any immune reactions to a blood transfusion, your doctor will carefully consider the possibility of Rh incompatibility as well as other blood group incompatibilities. When making the diagnosis, your doctor will also consider possible cell enzyme defects and hereditary blood disorders.

Treatment
Preventive treatment involves injections of Rh immune globulin (RhIg or RhoGAM). RhIg contains antibodies to the Rh factor. Those antibodies quickly attach to and help destroy Rh-positive fetal cells in the motheroduct, Rh immune globulin has not been shown to transmit any infectious diseases. This treatment is recommended for Rh-negative women who do not have antibodies to Rh-positive blood at 28 weeks of pregnancy and after any of the following events:
miscarriage or elective abortion
ectopic pregnancy
vaginal bleeding or abdominal trauma during pregnancy
amniocentesis
chorionic villus sampling
external rotation or turning of the baby
For the same women, preventive treatment may be given within 72 hours of delivery of an Rh-positive newborn. This will prevent sensitization in more than 95 percent of Rh-negative women. The goal is to help prevent antibody production that may cause subsequent incompatibility.

RhIg will not work for a mother who has already produced the antibodies against Rh-positive blood due to a prior pregnancy, miscarriage, abortion or blood transfusion. When a mother has the antibodies, the father can be tested to determine Rh factor. If he is Rh-negative, the baby will be Rh-negative and not at any risk for Rh disease. If the father is Rh-positive (or his Rh status is unknown), an amniocentesis can be done to determine if the fetus is Rh-positive or Rh-negative. If the baby is Rh-positive, levels of antibodies in the mothermine if immediate treatment is needed. If there is any blood cell breakdown, your baby may need a blood transfusion. This is usually recommended when the baby's hematocrit (a measure of the amount of entire blood volume that consists of red blood cells) drops below 25 to 30 percent. Labor may be induced early if tests confirm that your antibodies are destroying fetal blood cells and your infant is developing complications.


Complications
Umbilical blood sampling or intrauterine blood transfusions may threaten a pregnancy. Rh incompatibility may also lead to fetal distress that requires premature delivery. In some cases, high levels of bilirubin (a yellow pigment formed when red blood cells die) may lead to kernicterus, a severe form of jaundice that can lead to neurological damage, muscle rigidity or mental deficiency. In some cases, it can be fatal.
Erythroblastosis fetalis is also a possibility. This is a type of anemia in newborns of maternal-fetal blood incompatibilities that may involve fetal heart failure, swelling caused by excessive fluid, fluid accumulation in the peritoneal cavity and pericardial effusion (fluid between the membranes surrounding the heart). Fetal liver function may also be affected. In some cases, the fetus may not survive.


Prevention
In addition to the preventive treatment measures described above, pretransfusion testing can help ensure that Rh-negative patients receive Rh-negative blood components.
During pregnancy, thorough screening techniques, careful monitoring and prompt treatment can prevent or minimize the potential complications related to Rh incompatibility.

If you're Rh-negative and have delivered one Rh-positive baby but have not developed antibodies to the Rh factor, you'll be given an RhIg injection for each subsequent pregnancy.