Copied from the official RhoGAM® site:

Understanding what it means to be Rh-negative

You’ve recently learned that you are “Rh-negative”, and therefore your baby may be at risk for hemolytic disease of the newborn, also known as HDN. Before hearing the news from your doctor or nurse, no doubt your thoughts were filled with all the joyful anticipations of a new baby: picking out names, planning a nursery, buying infant clothes, and sharing sonogram images with relatives and friends. But now, unfamiliar terms such as Rh factor, antibodies, and HDN may have you feeling concerned, even scared. Don’t be!

There is a treatment called RhoGAM® Brand that, if administered properly, can virtually eliminate the risk of HDN; your doctor will be prescribing it for you. The company that makes RhoGAM® Brand is the same one that helped discover, develop, and deliver this lifesaving miracle 40 years ago: Ortho Clinical Diagnostics, part of the Johnson & Johnson Family of Companies.

What Does It Mean to Be Rh-Negative?

Being Rh-negative means you don't have a certain protein ("D antigen" or the Rh factor) on the surface of your red blood cells. If you do have it, you're Rh-positive. So the terms “Rh-negative” and “Rh-positive” are really just terms that further define what type of blood a person has, beyond the general blood type categories of A, B, AB, and O.

Being Rh-negative is neither good nor bad, but it can become a serious issue if you’re pregnant by or planning to have a child with a man who is Rh-positive.

In the United States, the population of Rh-negative individuals varies among ethnic groups. The highest prevalence is found among Caucasians (15%), followed by African Americans and Hispanics (8% each), and Asians and Native Americans (1% each). For the small percentage of people who are Rh-negative, their blood type causes no special health concerns except when they give or receive blood, or during pregnancy.


WHEN NEGATIVE AND POSITIVE MEET AND MATE

If you're Rh-negative and carrying a baby who is Rh-positive (like the baby’s father), your baby may be at risk for a serious disease. When your immune system (which fights off invaders to keep you healthy) is exposed to your baby’s Rh-positive blood, it will begin producing antibodies that are sensitized(designed specifically) to destroy these “foreign” blood cells.

If this is your first pregnancy, chances are everything will be fine. These sensitized antibodies are typically not dangerous until subsequent pregnancies, when they've had time to grow in strength and number. However, all subsequent pregnancies would be at great risk. That is why you must act now to prevent your immune system from being exposed to Rh-positive blood.


CONSEQUENCES OF HDN FOR UNPROTECTED BABIES

If your immune system is not held in check by a product such as RhoGAM® Brand, sensitized antibodies programmed to recognize and attack any Rh-positive blood cells are waiting for the next pregnancy. The antibodies then begin to destroy the red blood cells of the baby, leading to complications such as jaundice, anemia, mental retardation, and heart failure1. This condition, known as hemolytic disease of the newborn (HDN), can be fatal in the most severe cases—the infant would typically die during pregnancy or shortly after delivery.

Prior to the introduction of RhoGAM® Brand in 1968, HDN was a major cause of infant mortality, responsible for an average of 10,000 deaths annually in the United States2. Since the introduction of Rh immune globulin products, like RhoGAM® Brand, the incidence of HDN has been virtually eliminated in most developed countries.


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