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.

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 testing
• 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 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. He or she might be given a blood transfusion through the umbilical cord during the pregnancy or immediately after delivery if necessary.

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.