What is the Rh factor?
The Rh factor is a protein that some people have on the surface of their red blood cells.
If you have the protein, you're Rh-positive
If you don’t have the protein, you're Rh-negative
Being Rh-positive or Rh-negative doesn’t impact your health, but it can affect your pregnancy
Your Rh factor doesn’t change over time—you’re either positive or negative your whole life
What is Rh incompatibility?
Rh incompatibility is when you’re Rh-negative but your baby is Rh-positive
You don't have Rh incompatibility if you're Rh-positive and your baby is Rh-negative
The only way you can have Rh incompatibility is if:
You’re Rh-negative AND
Your baby’s father is Rh-positive AND
Your baby is Rh-positive
Why is Rh incompatibility a problem?
Rh incompatibility by itself isn't a problem. Rh incompatibility becomes a problem if some of your baby’s Rh-positive blood gets into your Rh-negative blood. Many people think that a baby's blood doesn't mix with the mother's blood. However, this mixing often happens when you:
Deliver a baby
Have a miscarriage
Have an abortion
Have a bad injury to your belly while pregnant
When your baby’s Rh-positive blood gets into your Rh-negative blood, your body's immune system reacts badly. Your immune system makes a protein called an Rh antibody to fight against your baby's Rh-positive blood. Those Rh antibodies can get into and destroy your baby's Rh-positive blood cells.
Rh incompatibility doesn't harm a first pregnancy because you won't have any Rh antibodies until after you deliver your first baby or have a miscarriage.
Rh incompatibility can harm a second (or later) pregnancy. If you have Rh antibodies from an earlier pregnancy, now your Rh-positive baby could have problems. Your Rh antibodies may destroy some of your baby's red blood cells and cause your baby to have:
Brain damage
The more times you've been pregnant with Rh incompatibility, the more Rh antibodies you have. The more Rh antibodies you have, the worse the problems for your next baby.
How can doctors tell if I have Rh incompatibility?
At your first doctor’s visit during a pregnancy, you'll have a blood test look for the Rh factor.
If you’re Rh-positive, there's no problem.
If you’re Rh-negative, your baby's father should have a blood test to look for the Rh factor:
If your baby's father is Rh-negative, there's no problem
If your baby's father is Rh-positive, your baby may be Rh-positive, causing Rh incompatibility
If you're Rh-negative and your baby's father hasn't been tested or if he's Rh-positive, you'll have blood tests throughout your pregnancy to look for Rh antibodies.
What will doctors do if I have Rh incompatibility?
You and your baby don't need any treatment if your body isn’t making too many Rh antibodies
If your blood tests show that your body is making a lot of Rh antibodies, you'll have more tests to see if your baby has anemia Anemia in the Newborn Anemia is a disorder in which there are too few red blood cells in the blood. Anemia can occur when red blood cells are broken down too rapidly, too much blood is lost, or the bone marrow does... read more (not enough healthy red blood cells). Those tests can include taking a sample of blood from the baby and having a special ultrasound Ultrasonography Ultrasonography is a safe imaging test that uses sound waves to create a moving picture of the insides of your body. Ultrasonography doesn't use radiation (x-rays). Ultrasonography is also called... read more 
(taking moving pictures of the insides of your uterus) to look at the blood flow in your baby's brain.
If your baby has anemia, your baby will get one or more blood transfusions before birth (and maybe even after birth)
Doctors often give you medicines called corticosteroids to help your baby's lungs grow
When your baby's lungs have grown enough to work well outside your body, doctors will artificially start (induce) labor
How can I prevent Rh incompatibility?
If you have Rh-negative blood, doctors will give you a shot of Rh0(D) immune globulin. This shot makes your body less able to react to your baby's Rh-positive blood. That makes you less likely to make Rh antibodies that can hurt your baby. You'll get these shots:
At 28 weeks of pregnancy
Within 72 hours after delivering a baby with Rh-positive blood, even after a miscarriage or abortion
After any vaginal bleeding
After getting certain tests (such as amniocentesis and chorionic villus sampling) when your baby's blood and your own blood can mix
