Rh is short for Rhesus factor. Rhesus factor is a type of antigen present on the surface of red blood cells. If you have have this particular antigen, you are Rh positive. If you don’t, you’re Rh negative. The Rh factor is important in determining blood type for transfusions.
Normally Your immune system protects against potentially harmful substances (antigens). When foreign antigens enter your body, your immune system makes antibodies to help destroy them. If you are Rh negative and you’re exposed to Rh-positive blood your body may make antibodies to Rh-positive blood on your next exposure to it.
If a mother is Rh negative and a father is Rh positive, the child will likely be Rh-positive. This can create problems with pregnancy. During pregnancy a mother may be exposed to the Rh positive cells of her baby and develop antibodies to them. During a future pregnancy, these antibodies can destroy the red blood cells of an unborn child, leading to serious problems. If both parents are Rh-negative, they can only have Rh-negative offspring. If the mother is Rh-positive, her body will not produce antibodies that will cause these problems.
To reduce this risk, an Rh-negative pregnant woman may be given injections of Rh immunoglobulin (RhIg). The injections are given at about 28 weeks in the pregnancy and immediately after delivery. With these injections, Rh-negative women can have safe Rh-positive pregnancies. RhIg is given to all Rh-negative women whose fetuses might be Rh-positive. The fetus may be Rh-positive if its father is Rh-positive. If both parents are Rh-negative, they can only have Rh-negative offspring.