Does this test have other names?

Rh incompatibility, Rh factor

What is this test?

Rhesus (Rh) typing is used to find out if you have a certain protein called Rh factor on the outer layer of your red blood cells. If you don't 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 make antibodies against it. This can cause a miscarriage, anemia, and problems in later pregnancies. The first pregnancy is often 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, miscarriage, or other instances when you may have been exposed to Rh positive blood. 

Why do I need this test?

Pregnant people typically have this test at their first prenatal appointment. You may need this test if you:

• Are pregnant

• Are thinking about becoming pregnant

• Are the father of an unborn baby

• Are Rh negative and having a baby with a male 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. If antibodies are present, it 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 can't or won't have the Rh typing blood test. During amniocentesis, a needle is put into the uterus to remove a small amount of amniotic fluid for testing.

What do my test results mean?

Test results may vary depending on your age, gender, health history, and other things. Your test results may be different depending on the lab used. They may not mean you have a problem. Ask your healthcare provider what your test results mean for you.

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 is done with a blood sample. A needle is used to draw blood from a vein in your arm or hand. 

Does this test pose any risks?

Having a blood test with a needle carries some risks. These include bleeding, infection, bruising, and feeling lightheaded. When the needle pricks your arm or hand, you may feel a slight sting or pain. Afterward, the site may be sore. 

What might affect my test results?

Nothing is likely to affect your test results. This includes your diet, lifestyle, and medicines.

How do I get ready for this test?

A blood test rarely needs any preparation. You can likely eat, drink, and take any medicine as usual, but check with your healthcare provider. Be sure your healthcare provider knows about all medicines, herbs, vitamins, and supplements you are taking. This includes medicines that don't need a prescription and any illegal drugs you may use.