What is Rh factor incompatibility in pregnancy?

Rh factor incompatibility in pregnancy is a medical condition that occurs when the mother is Rh-negative (Rh-) and the fetus is Rh-positive (Rh+). The Rh factor is a protein found on the surface of red blood cells, which is inherited from parents [1]. In this situation, the mother's body may recognize the fetus's Rh-positive blood cells as a foreign substance, leading to the production of antibodies against them [2].

Usually, these antibodies do not cause problems in the first pregnancy because the mother's body does not have enough time to produce a large amount of them. However, in subsequent pregnancies with an Rh-positive fetus, the previously formed antibodies can cross the placenta and attack the fetus's red blood cells, which may lead to severe anemia and other serious health problems, and in some cases, it may be fatal [1].

How are Rh antibodies formed?

Maternal and fetal blood do not usually mix directly during pregnancy. However, minor mixing of blood can occur under certain circumstances, which stimulates the mother's body to produce antibodies. These circumstances include childbirth, miscarriage, ectopic pregnancy, or induced abortion. Blood mixing can also occur during medical procedures such as amniocentesis or chorionic villus sampling (CVS), or in cases of bleeding during pregnancy, abdominal trauma, or manual attempts to turn the fetus from a breech position [1].

The importance of early detection and prevention

Early detection of the mother's Rh factor is vital to prevent complications. The mother's blood type and Rh status are typically determined through a simple blood test at the start of prenatal care [1]. If the mother is Rh-negative, an antibody screening test is performed to confirm whether she has already developed antibodies [2].

The goal of treatment is to prevent the Rh-negative mother from developing Rh antibodies in the first place. This is done by administering a medication called Rh immunoglobulin (RhIg), which stops the body from producing these antibodies [1]. This medication is not effective if the mother has already formed antibodies [1].

When is the Rh immunoglobulin (RhIg) injection given?

Administering a dose of Rh immunoglobulin (RhIg) is recommended in several situations to prevent antibody formation [1]:

• At about 28 weeks of pregnancy, as a small percentage of Rh-negative women may be exposed to fetal Rh-positive blood cells in the final months of pregnancy [1].
• Within 72 hours after the birth of an Rh-positive baby, as the chance of fetal blood mixing with maternal blood is greatest during labor [1].
• After a miscarriage or induced abortion, especially after 12 weeks of pregnancy [1].
• After an ectopic pregnancy, amniocentesis, chorionic villus sampling, fetal blood sampling, or fetal surgery [1].
• In the event of bleeding after 20 weeks of pregnancy or abdominal trauma during pregnancy [1].
• If a manual attempt is made to turn the fetus from a breech position [1].

It is important to note that Rh immunoglobulin treatment is only effective for the pregnancy for which it is administered. Every pregnancy and birth of an Rh-positive baby requires a repeat dose of RhIg [1]. For more information about the Rh factor prevention shot (Arabic), you can visit our dedicated page.

What happens if antibodies have already formed?

If an Rh-negative mother has already developed antibodies, Rh immunoglobulin treatment will not be useful [1]. In this case, the fetus's health is closely monitored throughout the pregnancy, usually using ultrasound scans [1]. If tests show that the fetus has severe anemia, early delivery (before 37 weeks of pregnancy) or performing a blood transfusion for the fetus through the umbilical cord while still in the womb may be necessary [1]. In cases of mild anemia, delivery may occur at the normal time, and the baby may need a blood transfusion or other treatments after birth, such as phototherapy, if they suffer from jaundice [2].

Medical references

• The Rh Factor: How It Can Affect Your Pregnancy
• Rh Incompatibility
• Fetal-maternal erythrocyte distribution blood test