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 (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- 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 mild to serious anemia in the fetus.

This test determines the amount of blood that has been exchanged between the mother and fetus. All Rh- pregnant women should receive this test when bleeding or risk of bleeding events occur during a pregnancy.

In a woman whose blood is ABO incompatible with her infant, this test helps determine the amount of Rh immune globulin (RhoGAM) an Rh-negative woman must receive to prevent her from developing abnormal proteins that attack the fetus in future pregnancies.

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 means there blood from the fetus is leaking into the mother's blood circulation. The greater the amount of fetal cells present, the greater the amount of Rh immune globulin the mother must receive.

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