Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.
Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.
The sample is taken to the laboratory, where the hemoglobin is removed and both the baby's and mother's red blood cells are counted.
How to prepare for the test
No special preparation is necessary for this test.
How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed
Rh incompatibility occurs when the mother's blood type is Rh-negative (Rh-) and her unborn baby's blood type is Rh-positive (Rh+). If the mother is Rh+, or if both parents are Rh-, there is no reason to worry about Rh incompatibility.
If the baby's blood is Rh+ and gets into the mother's Rh- bloodstream, her body will produce antibodies. These antibodies could pass back through the placenta and harm the developing baby's red blood cells. This can cause mild to serious anemia in the unborn baby.
This test determines the amount of blood that has been exchanged between the mother and fetus. All Rh- pregnant women should get this test if they have bleeding or a risk of bleeding during the pregnancy.
In a woman whose blood is Rh incompatible with her infant, this test helps determine how much Rh immune globulin (RhoGAM) she must receive to prevent her body from producing abnormal proteins that attack the unborn baby in future pregnancies.
In a normal value, no or few of the baby's cells are in the mother's blood. The standard dose of RhoGAM is enough in this case.
Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
What abnormal results mean
In an abnormal test result, blood from the unborn baby is leaking into the mother's blood circulation. The more of the baby's cells there are, the more Rh immune globulin the mother must receive.
What the risks are
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight, but may include:
Infection (a slight risk any time the skin is broken)
Neil K. Kaneshiro, MD, MHA, Clinical Assistant Professor of Pediatrics, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.