A+, A-, B+, B-. You probably haven't seen more plus and minus signs since high school or your last series of home pregnancy tests.

What is the "+" or "-" sign after a blood type of A, B, O or AB? It is indicative of an Rh factor and it is critical that an expectant mother and her health care provider are aware of an Rh-negative blood factor. It's not critical for the mother's sake, but for the sake of her unborn child or future unborn children. The prenatal blood screen performed during the first prenatal visit will determine Rh status, with results available within one to two business days and no risk to mother or fetus.

A blood type with a positive Rh factor will produce a protein that inhibits the surface of red blood cells. Rh-negative red blood cells lack this protein. The Rh factors were named after they were first discovered in the blood of the rhesus monkey, according to the Encarta encyclopedia.

If an Rh-negative woman and an Rh-positive man conceive a child together, the fetus' Rh factor will be positive or negative, with no harm to an Rh-negative baby and most likely no harm to a Rh-positive baby. However, if the baby's Rh factor is positive, during delivery the fetus' Rh-positive blood cells may enter the mother's bloodstream. In very rare situations, this can happen during pregnancy. Since an Rh-negative mother's bloodstream does not contain the Rh factor, her body will produce antibodies to fight off the foreign red blood cells, thus threatening the red blood cells of future unborn children. If an Rh-negative mother conceives an Rh-negative baby, there are no foreign red blood cells present and no threat of future fetal blood cell destruction.

An injection to protect the fetuses of Rh-negative mothers was developed in 1968. An injection of Rh immunoglobulin (Rhlg) given to the mother during the 28th week of pregnancy can usually prevent harm to a developing fetus. This injection is commonly known as a Rhogam injection. If the baby is born with an Rh-positive factor, the mother will receive a second injection within 72 hours after birth to protect the next baby. If the new baby is Rh-negative, no second injection is necessary. The injection is given via the mother's hip and it might sting a bit. An injection is also given to Rh-negative mothers who miscarry.

Jay M. Kulkin, doctor and medical director of BlueCross/BlueShield of Georgia, says, "Rh disease still exists though it is uncommon. Occasionally, Rh-negative women will miscarry and not be aware that the bleeding they are doing is from a miscarriage. There are many other complex reasons women may become Rh sensitized. The most important issue is for Rh-negative women to receive Rhogam as recommended by their doctor during any pregnancy."

Darlene Manus of Paramount, California has a negative Rh factor. She gave birth to her first child in the early 1960s before the Rhlg injection was available. She was relieved to find out that her baby was also born Rh-negative. This meant no impending risk to a second baby in her future.

By the time Darlene became pregnant with her second child in 1968, the Rhlg injection was available. She was thankful for the injection because her second child was Rh-positive, and without the injections, her third baby could have been harmed when she was born in 1970.

Darlene's only Rh-negative baby girl grew up to be a mother of four. Only one of her four children is Rh-negative and there were no complications for the Rh-positive children . We can thank medical technology for the Rh compatibility possibilities of today.