The fetal-maternal erythrocyte distribution test is used to measure the number of fetal red blood cells in a pregnant woman's blood. (See also Rh incompatibility.)

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. 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 fetal and maternal red blood cells are counted. Based on that count a ratio of fetal to maternal blood cells is determined.

No special preparation is necessary for this test.

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.

Rh incompatibility occurs when the mother's blood type is Rh-negative and her unborn baby's blood type is Rh-positive. If the mother is Rh-positive, or if both parents are Rh-negative, there is no reason to worry about Rh incompatibility.

However, if the mother is Rh-negative and the baby's father is Rh-positive, then most likely the baby will inherit the father's blood type, creating incompatibility between mother and fetus. In this case, if some of the baby's blood gets into the mother's blood stream, her body will produce antibodies. These antibodies could pass back through the placenta and harm the developing baby's red blood cells, causing very mild to very serious anemia in the fetus.

Rh disease is much more common in second and subsequent Rh-incompatible pregnancies, because the mother's and baby's blood usually do not mix until delivery.

This test determines the amount of blood that has been exchanged between the mother and fetus. All Rh-negative pregnant women should receive this test during their first prenatal visit, and subsequent visits as needed.

This test also helps determine how much of a drug called RhoGAM should be administered. RhoGAM will attack the fetal cells in the mother's blood stream and prevent her from developing antibodies in response to her baby's blood.

A normal value indicates that no or few fetal cells are present in the maternal blood. The standard dose of RhoGAM is sufficient in this case.

A high fetal-maternal red blood cell ratio indicates a need for additional RhoGAM.

The risks associated with drawing blood are slight, but include:

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