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, problems can occur if you're Rh negative and the baby you're carrying is Rh positive. Usually, your blood does not mix with your baby's blood during pregnancy. However, during delivery or certain times during pregnancy, 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 proteins called Rh antibodies after exposure to the baby's red blood cells. The antibodies produced aren't a problem during the first pregnancy. The concern is with your next pregnancy. If your next baby is Rh positive again, your body will produce Rh antibodies that can cross the placenta and damage the baby's red blood cells. This could lead to life-threatening anemia, a condition in which there are not enough red blood cells. If this condition is not prevented, each Rh positive baby you carry after your first pregnancy might have severe anemia.

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.

If you're Rh negative and your baby might be or is Rh positive, you'll also need an Rh immune globulin injection after any situation in which your blood could come into contact with the baby's 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
  • Fetal blood sampling

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.

Mother's Rh factor Father's Rh factor Baby's Rh factor Precautions
Rh positive Rh positive Rh positive None
Rh negative Rh negative Rh negative None
Rh positive Rh negative Could be Rh positive or Rh negative None
Rh negative Rh positive Could be Rh positive or Rh negative Rh immune globulin injections
June 23, 2015