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The Rh factor test is done during pregnancy to identify a woman's Rh factor. In some cases, the baby's father might need an Rh factor test, too.

During pregnancy — or, more likely, during delivery — a small amount of your baby's blood could come in contact with your blood. If you're Rh positive, Rh incompatibility isn't a concern. If you're Rh negative and your baby is Rh positive, however, your body might produce Rh antibodies after exposure to the baby's red blood cells.

Typically, the antibodies aren't a problem during the first pregnancy. The concern is with a subsequent pregnancy with an Rh positive baby. In this case, your existing Rh antibodies might cross the placenta and fight the baby's red blood cells. This could lead to life-threatening anemia — a lack of healthy red blood cells — for the Rh positive baby.

If you're Rh negative, you might need to have another blood test — an antibody screen — during your first trimester and again during week 28 of pregnancy. The antibody screen is used to detect antibodies to Rh positive blood.

If you haven't started to produce Rh antibodies, you'll need an injection of a blood product called Rh immune globulin. The immune globulin prevents your body from producing Rh antibodies during your pregnancy.

If your baby is born Rh negative, no additional treatment is needed.

If your baby is born Rh positive, you'll need another injection shortly after delivery. You'll likely need injections during any subsequent pregnancies and after the delivery of each Rh positive baby as well.

If you're Rh negative, you'll also need an Rh immune globulin injection after any situation in which your blood could come into contact with Rh positive blood, including:

• Miscarriage
• Abortion
• Ectopic pregnancy — when a fertilized eggs implants somewhere outside the uterus, usually in a fallopian tube
• Molar pregnancy — a noncancerous (benign) tumor that develops in the uterus
• Amniocentesis — a prenatal test in which a sample of the fluid that surrounds and protects a baby in the uterus (amniotic fluid) is removed for testing or treatment
• Chorionic villus sampling — a prenatal test in which a sample of the wispy projections that make up most of the placenta (chorionic villi) is removed for testing
• Bleeding during pregnancy
• Blunt trauma to the abdomen during pregnancy
• Rotation of a baby in a breech position — such as buttocks first — before labor

If the antibody screen shows that you're already producing antibodies, an injection of Rh immune globulin won't help. Your baby will be carefully monitored. If necessary, he or she might be given a blood transfusion through the umbilical cord during pregnancy or immediately after delivery.