Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill 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 tourniquet 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.

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.

Some red blood cells from the fetus are transferred into the mother's bloodstream during a miscarriage, abortion, amniocentesis, or normal delivery. If a large number of red blood cells are transferred to an from an Rh-positive fetus to an Rh-negative mother, Rh sensitization may occur. If the mother is Rh negative and the fetus is Rh positive, the mother's body may produce antibodies against the fetal cells, and this could harm her next child during pregnancy.

All Rh-negative pregnant women should be screened during their first prenatal visit, and subsequent visits as indicated by the health care provider. RhoGAM is a drug given to all Rh-negative mothers whenever there is a risk of maternal and fetal blood exchange during an Rh-positive pregnancy.

This test determines the amount of maternal-fetal blood exchange, and estimates the quantity of RhoGAM that should be administered. RhoGAM will attack the fetal cells in the maternal bloodstream and prevent maternal formation of anti-Rh antibodies that could endanger 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.

An increased fetal-maternal red blood cell ratio indicates a need for additional RhoGAM.

• 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