Does this test have other names?
Rh incompatibility, Rh factor
What is this test?
Rhesus (Rh) typing is used to determine whether you have a specific protein called Rh factor on the outer layer of your red blood cells. If you do not have Rh factor in your blood, you are Rh negative. If you do have Rh factor in your blood, you are Rh positive. Most people are Rh positive, but Rh negative blood types can be passed down from parents to children.
Rh typing is important during pregnancy. If you are Rh negative and your baby is Rh positive, you may have an Rh incompatibility. During pregnancy, it's possible that some of your baby's blood could pass through the placenta into your body. Your body may react to the baby's blood as a foreign substance and produce antibodies against it. This can cause a miscarriage, anemia, and problems in later pregnancies. The first pregnancy is usually not affected by Rh incompatibility because the baby is often born before many of the antibodies develop. These risks can be lowered by a shot or shots of Rh immunoglobulin during each pregnancy.
Why do I need this test?
Pregnant women typically have this test at their first prenatal appointment. You may need this test if you:
Are thinking about becoming pregnant
Are the father of an unborn baby
Are Rh negative and having a baby with a man who is Rh positive
Need a blood transfusion
Are donating blood
What other tests might I have along with this test?
If you have an Rh typing test and the results show you are Rh negative, you may need to have another blood test called an antibody screen. The antibody screen looks for antibodies in your blood. This means that you have been exposed to Rh positive blood and developed antibodies to it. You may need multiple antibody screens during your pregnancy and one at the time of delivery.
You may also need an amniocentesis if the baby's father cannot or will not have the Rh typing blood test. During amniocentesis, a needle is inserted into the uterus to remove a small amount of amniotic fluid for testing.
What do my test results mean?
Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.
Having Rh factor in your blood means you are Rh positive. If you do not have Rh factor in your blood, you are Rh negative. If you are Rh negative and having a baby with a man who is Rh positive, you could be at risk for pregnancy complications.
How is this test done?
The test requires a blood sample, which is drawn through a needle from a vein in your arm.
Does this test pose any risks?
The risks are minor. The finger prick or needle may feel uncomfortable or painful. You may experience bruising, soreness, or pain in your hand or arm at the puncture spot. These symptoms usually go away soon after the test is over.
What might affect my test results?
Nothing is likely to affect your test results. This includes your diet, lifestyle, and medications.
How do I get ready for this test?
A blood test rarely requires any preparation. You can probably eat, drink, and take your medication as usual, but check with your doctor to be sure. Remember to tell your doctor about any medications or supplements that you take.
- Haines, Cynthia, MD
- Hanrahan, John, MD