A blood sample is needed.

No special preparation is necessary for this test.

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.

Rh incompatibility is a condition that 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 find out 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 Rh immune globulin is enough in this case.

Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or test different samples. Talk to your health care provider about the meaning of your specific test results.

An abnormal test result means that 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.

There is little risk involved with having your blood taken. Veins and arteries vary in size from one person to another and from one side of the body to the other. Taking blood from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight, but may include:

• Excessive bleeding
• Multiple punctures to locate veins
• Fainting or feeling lightheaded
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

Kleihauer-Betke stain; Flow cytometry - fetal-maternal erythrocyte distribution; Rh incompatibility - erythrocyte distribution

Cooling L, Downs T. Immunohematology. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 36.

Moise KJ. Red cell alloimmunization. In: Landon MB, Galan HL, Jauniaux ERM, et al, eds. Gabbe's Obstetrics: Normal and Problem Pregnancies. 8th ed. Philadelphia, PA: Elsevier; 2021:chap 40.

Updated by: Charles I. Schwartz, MD, FAAP, Clinical Assistant Professor of Pediatrics, Perelman School of Medicine at the University of Pennsylvania, General Pediatrician at PennCare for Kids, Phoenixville, PA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.