Most people have Rh-positive blood, meaning that they produce the "Rh factor," an inherited protein found on the surface of their red blood cells. But about 15 percent of the white population and 7 percent of the African-American population lack the Rh factor. These people are Rh-negative.

The health of an Rh-negative person is not affected in any way. However, an Rh-negative woman is at risk of having a baby with Rh disease if she conceives a baby with an Rh-positive father and the baby inherits the father's Rh-positive blood type.

What is Rh disease?
Rh disease is a condition caused by an incompatibility between the blood of a mother and that of her fetus. If the mother is Rh-negative and her baby is Rh-positive, during pregnancy (and especially during labor and delivery) some of the fetus's Rh-positive red blood cells may get into the mother's bloodstream. Since these red blood cells are foreign to the mother's system, her body will try to fight them off by producing antibodies against them — a process called "sensitization."

In a first pregnancy, there's very little danger to an Rh-positive baby because the child is usually born before the mother produces substantial Rh antibodies. However, since the woman will continue to produce antibodies throughout her life, during her next pregnancy, maternal Rh antibodies can cross the placenta and reach the fetus.

This is a disease that destroys an unborn baby's blood cells, potentially resulting in newborns being born with jaundice (yellowing of the skin and eyes) and anemia. In some cases the results could be brain damage, heart failure, and even death. But today, doctors are able to detect and treat Rh disease in the fetus, so about 95 percent of babies with severe Rh disease survive.

How common is Rh disease?
While the disease once affected 20,000 babies in the U.S. each year, fortunately it's rare today. Since 1968, a treatment that prevents Rh disease (injections of a blood product called Rh immunoglobulin) has dramatically reduced the number of babies born with it.

But not all women who need the treatment get it, and a small number of women can't be helped by the injections. As a result, some 4,000 babies still develop Rh disease each year. The good news is that the disease can be treated, and doctors are now able to manage it earlier than ever before, often even before birth. And in some cases Rh disease is so mild that it doesn't require treatment.