Rh incompatibility can occur when a mother's Rh factor is negative and her fetus's Rh factor is positive. Rh factor is a protein on red blood cells and is indicated after a person's blood type as being either positive or negative. For example, if a woman's blood type is AB- then she has a negative Rh factor. This can be of concern during pregnancy if the fetus has a positive RH factor. If, at any point during the pregnancy, some of the unborn baby's blood mixes with the mother's blood, the woman's immune system will see the baby's positive Rh protein as foreign and create antibodies against it. This usually doesn't lead to any problems with the first pregnancy, as few antibodies are created by the time the baby is born, but can create issues with any subsequent pregnancies if that baby's Rh factor is also positive, as the woman's body has had time to build up antibodies. This can lead to complications if not treated with an injection called Rh immunoglobulin (RhIg), which prevents the antibodies from being formed.