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Rh factor blood test

Overview

Rh factor is an inherited protein found on the surface of red blood cells. If your blood has the protein, you're Rh positive. If your blood doesn't have the protein, you're Rh negative. The "+" or "–" you might see after your blood type refers to Rh positive or Rh negative.

Rh positive is much more common than Rh negative. Having an Rh negative blood type is not an illness, and it usually does not affect your health. But it can affect pregnancy. Your pregnancy needs special care if you're Rh negative and your baby is Rh positive. That's called Rh incompatibility. A baby can inherit the Rh factor from either parent.

Your health care provider will advise that you have a blood type and Rh factor screening test during your first prenatal visit. This will show whether you are Rh positive or Rh negative.

Why it's done

During pregnancy, problems can happen if you're Rh negative and your baby is Rh positive. Usually, your blood doesn't mix with your baby's blood during pregnancy. However, a small amount of your baby's blood could come in contact with your blood when the baby is born. It can also happen if you have bleeding or trauma to your abdomen during pregnancy.

If you're Rh negative and your baby is Rh positive, your body might produce proteins called Rh antibodies if your blood and the baby's blood mix. Those antibodies aren't a problem during the first pregnancy. But problems can happen if you become pregnant again.

If your next baby is Rh positive, the Rh antibodies can cross the placenta and damage the baby's red blood cells. This could lead to life-threatening anemia, a condition in which red blood cells are destroyed faster than the baby's body can replace them. Red blood cells are needed to carry oxygen throughout the body.

If you're Rh negative, you might need to have another blood test — called an antibody screen — several times: during your first trimester, during week 28 of pregnancy and when your baby is born. Some people need the test more often.

That test is used to detect antibodies to Rh positive blood. If you haven't started to produce Rh antibodies, you'll likely need a shot (injection) of a blood product called Rh immune globulin. This prevents your body from producing Rh antibodies during your pregnancy.

If your baby is born Rh negative, you don't need any other treatment. 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, your health care provider may recommend an Rh immune globulin injection after situations in which your blood could come into contact with the baby's blood, including:

  • Miscarriage
  • Ectopic pregnancy — when a fertilized eggs implants somewhere outside the uterus, usually in a fallopian tube
  • Abortion
  • Removal of a 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
  • Cordocentesis — a prenatal test in which a sample of the baby's blood is removed from the umbilical cord for testing
  • Bleeding during pregnancy
  • Injury or other trauma to your abdomen during pregnancy
  • The external manual rotation of a baby in a breech position — such as buttocks first — before labor
  • Delivery

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 during your pregnancy. The baby might be given a blood transfusion through the umbilical cord during pregnancy or immediately after delivery if necessary.

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

What you can expect

An Rh factor test is a basic blood test. The blood sample usually is taken during the first prenatal visit and sent to a lab for testing. No special preparation is necessary.

Results

If you're Rh positive, you don't need to do anything.

If you're Rh negative and your baby is Rh positive, your body could make antibodies that might be harmful during another pregnancy. Take these steps:

  • If you have vaginal bleeding any time during pregnancy, contact your health care provider right away.
  • Talk with your health care provider about scheduling an Rh immune globulin injection during your pregnancy.
  • Remind your health care team during labor that you're Rh negative.
Last Updated: July 29th, 2022