Rh positive refers to the presence of the Rh factor, an inherited protein found on the surface of red blood cells. If a persons blood has the Rh protein, they are Rh positive, and if their blood does not have the protein, they are Rh negative. Rh positive is much more common than Rh negative, with approximately 94% of the worldwide population having Rh-positive blood. Rh factor is important to determine in blood donations and transfusions, as a person with Rh-positive blood can receive both Rh-positive and Rh-negative transfusions, but those with Rh-negative blood can only receive Rh-negative blood. During pregnancy, Rh incompatibility can occur if an Rh-negative woman becomes pregnant with an Rh-positive fetus. This can lead to serious health problems for the fetus or newborn, as the mothers body may recognize the Rh-positive blood as foreign and produce antibodies against it. Knowing ones Rh status is important during pregnancy to prevent potential complications.