The Rh blood group is one of the most complex blood groups known in humans. From its discovery 60 years ago where it was named (in error) after the Rhesus monkey, it has become second in importance only to the ABO blood group in the field of transfusion medicine. It has remained of primary importance in obstetrics, being the main cause of hemolytic disease of the newborn (HDN).
The complexity of the Rh blood group antigens begins with the highly polymorphic genes that encode them. There are two genes, RHD and RHCE, that are closely linked. Numerous genetic rearrangements between them has produced hybrid Rh genes that encode a myriad of distinct Rh antigens. To date, 49 Rh antigens are known.
The significance of the Rh blood group is related to the fact that the Rh antigens are highly immunogenic. In the case of the D antigen, individuals who do not produce the D antigen will produce anti-D if they encounter the D antigen on transfused RBCs (causing a hemolytic transfusion reaction, HTR) or on fetal RBCs (causing HDN). For this reason, the Rh status is routinely determined in blood donors, transfusion recipients, and in mothers-to-be.
Despite the importance of the Rh antigens in blood transfusion and HDN, we can only speculate about the physiological function of the proteins, which may involve transporting ammonium across the RBC membrane and maintaining the integrity of the RBC membrane.
How Do I Know If I Am Rh Negative Or Rh Positive?
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. When the mother is Rh-negative and the father is Rh-positive, the fetus can inherit the Rh factor from the father. This makes the fetus Rh-positive too. Problems can arise when the fetus’s blood has the Rh factor and the mother’s blood does not.
Rh factor blood test
The Rh factor test is done during pregnancy to identify a woman’s Rh factor. In some cases, the baby’s father might need an Rh factor test, too.
During pregnancy, problems can occur if you’re Rh negative and the baby you’re carrying is Rh positive. Usually, your blood does not mix with your baby’s blood during pregnancy. However, during delivery or certain times during pregnancy, a small amount of your baby’s blood could come in contact with your blood.
If you’re Rh positive, Rh incompatibility isn’t a concern. If you’re Rh negative and your baby is Rh positive, however, your body might produce proteins called Rh antibodies after exposure to the baby’s red blood cells. The antibodies produced aren’t a problem during the first pregnancy. The concern is with your next pregnancy. If your next baby is Rh positive again, your body will produce Rh antibodies that can cross the placenta and damage the baby’s red blood cells. This could lead to life-threatening anemia, a condition in which there are not enough red blood cells. If this condition is not prevented, each Rh positive baby you carry after your first pregnancy might have severe anemia.
If you’re Rh negative, you might need to have another blood test — an antibody screen during your first trimester and again during week 28 of pregnancy. The antibody screen is used to detect antibodies to Rh positive blood.
If you haven’t started to produce Rh antibodies, you’ll need an injection of a blood product called Rh immune globulin. The immune globulin prevents your body from producing Rh antibodies during your pregnancy.
If your baby is born Rh negative, no additional treatment is needed. If your baby is born Rh positive, you’ll need another injection shortly after delivery.
If you’re Rh negative and your baby might be or is Rh positive, you’ll also need an Rh immune globulin injection after any situation in which your blood could come into contact with the baby’s blood, including:
- Miscarriage
- Abortion
- 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
If the antibody screen shows that you’re already producing antibodies, an injection of Rh immune globulin won’t help. Your baby will be carefully monitored. If necessary, he or she might be given a blood transfusion through the umbilical cord during pregnancy or immediately after delivery.
| Mother’s Rh factor | Father’s Rh factor | Baby’s Rh factor | Precautions |
|---|---|---|---|
| Rh positive | Rh positive | Rh positive | None |
| Rh negative | Rh negative | Rh negative | None |
| Rh positive | Rh negative | Could be Rh positive or Rh negative | None |
| Rh negative | Rh positive | Could be Rh positive or Rh negative | Rh immune globulin injections |
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