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Why it's done
By Mayo Clinic staffThe Rh factor test is done during pregnancy to identify a woman's Rh factor. In some cases, the baby's father may need an Rh factor test, too.
During pregnancy — or, more likely, during delivery — some of your baby's red blood cells may mix 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 may produce Rh antibodies after exposure to the baby's red blood cells. The antibodies are often not a problem during the first pregnancy. However, if you have a subsequent pregnancy with an Rh positive baby, your Rh antibodies may cross the placenta and attack the baby's red blood cells. The Rh positive baby then may develop Rh disease, a life-threatening condition that could cause anemia or other serious problems.
If you're Rh negative, you may 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 producing antibodies, you'll need to have an injection of a blood product (Rh immune globulin) in the muscle of your arm or buttocks to prevent your body from producing Rh antibodies during your pregnancy. Common side effects include soreness at the injection site or a slight fever. If your baby is born Rh negative, no additional action is needed. If your baby is born Rh positive, you'll need another injection shortly after delivery. You'll likely need injections during any subsequent pregnancies and after the delivery of each Rh positive baby as well.
If you're Rh negative, you'll also need an Rh immune globulin injection after any situation in which your blood could come into contact with Rh positive blood, including:
- Miscarriage
- Abortion
- Ectopic pregnancy
- Molar pregnancy
- Amniocentesis
- Chorionic villus sampling
- Bleeding during pregnancy
- Blunt trauma to the abdomen during pregnancy
- Rotation of a baby in breech position before labor
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 may 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 |
- Moise KJ. Management of Rhesus (Rh) alloimmunization in pregnancy. www.uptodate.com/home/index.html. Accessed Jan. 8, 2010.
- Moise KJ. Pathogenesis and prenatal diagnosis of Rhesus (Rh) alloimmunization. www.uptodate.com/home/index.html. Accessed Jan. 8, 2010.
- Moise KJ. Prevention of Rh (D) alloimmunization. www.uptodate.com/home/index.html. Accessed Jan. 8, 2010.
- The Rh factor: How it can affect your pregnancy. American College of Obstetricians and Gynecologists. http://www.acog.org/publications/patient_education/bp027.cfm. Accessed Jan. 19, 2010.
- Moise KJ. Management of rhesus alloimmunization in pregnancy. Obstetrics and Gynecology. 2008;112:164.
- McInerny TK, ed. American Academy of Pediatrics Textbook of Pediatric Care. Elk Grove Village, Ill.: American Academy of Pediatrics; 2009:941.
- Gabbe SG, et al. Obstetrics: Normal and Problem Pregnancies. 5th ed. Philadelphia, Pa.: Saunders Elsevier; 2007. http://www.mdconsult.com/das/book/body/179151994-3/941825473/1528/319.html#4-u1.0-B978-0-443-06930-7..50032-3--cesec8_1443. Accessed Jan. 20, 2010.
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