The determination of maternal Rh status is important in a woman with threatened miscarriage.

The Rh is a part of a person's blood type. Blood from the mother can get into the infant's bloodstream during an a miscarriage. If the mother has a Rh-negative blood type, she can form antibodies that destroy the blood cells of the fetus who has a Rh-positive blood type. The destruction of the infant's red blood cells can result in severe anemia, called erythroblastosis fetalis.

A medicine called RhoGAM is used to help prevent this problem. RhoGAM is an antibody that can prevent Rh-negative mothers from developing her own antibodies against Rh factor. Rh-negative pregnant women with vaginal bleeding will need an injection of RhoGAM to block the process.