Yet another test that's conducted on a pregnant woman's blood (and sometimes the father's blood, too) early in the pregnancy, the Rh (or Rhesus) factor test looks for a type of protein found on the surface of the red blood cells. If you have the protein -- as does approximately 85 percent of the population -- you're considered Rh positive and there is no potential issue [source: kidshealth.org]. If however, you lack the protein (Rh negative) and the father has it, complications can occur.
This is because the baby can inherit Rh-positive blood from the father. If this happens and then a baby's blood mixes with the mother's, the mother's body could treat the baby like something to which it is allergic, and the immune system could attack the baby's blood, causing a condition in the child known as hemolytic anemia.
It is rare for the blood of the fetus to mingle with the mother's blood during gestation however, so this condition is often a concern for second pregnancies. That's because intermingling of blood can occur during the first delivery and the next time an Rh-negative mother gets pregnant with an Rh-positive child, her antibodies will attack the fetus. This can be prevented by a simple vaccination around the 28th week of the pregnancy and again 72 hours after delivery of the first child.