Rh factor blood test

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See Also: Rh Factor Incompatibility

Rh factor blood test

Rhesus (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 lacks the protein, you're Rh negative.

Rh positive is the most common blood type. Having an Rh negative blood type is not an
illness and usually does not affect your health. However, it can affect your pregnancy.
Your pregnancy needs special care if you're Rh negative and your baby is Rh positive
(Rh incompatibility). A baby can inherit the Rh factor from either parent.
Your health care provider will recommend a blood type and Rh factor screening test
during your first prenatal visit. This will identify whether your blood cells carry the Rh
factor protein.

Why it's done
During pregnancy, problems can occur if you're Rh negative and the baby you're carrying
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
during delivery or if you experience bleeding or abdominal trauma during pregnancy. If
you're Rh negative and your baby is Rh positive, 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, these 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 — an antibody screen —
during your first trimester, during week 28 of pregnancy and at delivery. The antibody
screen is used to detect antibodies to Rh positive blood. If you haven't started to
produce Rh antibodies, you'll likely 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, your health care provider
might recommend an Rh immune globulin injection after situations 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
  • 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
  • Cordocentesis — a diagnostic prenatal test in which a sample of the baby's blood is
         removed from the umbilical cord for testing

  • Bleeding during pregnancy
  • Abdominal trauma during pregnancy
  • The external manual rotation of a baby in a breech position — such as buttocksfirst — 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. He or she might be
given a blood transfusion through the umbilical cord during the 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                                     None
                                                             positive or Rh
Rh negative                                          Rh positive                                       Rh immune
                                                             Could be Rh                                     globulin injections
                                                             positive or Rh

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

If you're Rh positive, no action is needed.
If you're Rh negative and your baby is Rh positive, there's a potential for your body to
produce antibodies that could be harmful during a subsequent pregnancy. If you have
vaginal bleeding at any time during pregnancy, contact your health care provider
immediately. Also, talk with your health care provider about scheduling an Rh immune
globulin injection during your pregnancy and remind your health care team of your Rh
status during labor.

Mayo Clinic


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