The Rh factor is a protein found on the surface of red blood cells. Most people who have the Rh factor are Rh-positive. Those who do not have the Rh factor are Rh-negative. At some point in your early pregnancy, you will have blood tests to find out your blood type. When the mother is Rh-negative and the father is Rh-positive, the fetus can inherit the Rh factor from the father. This makes the fetus Rh-positive too. 

This becomes an issue when the fetus’s blood has the Rh factor and the mother’s blood does not. If you are Rh-negative, and a small amount of the baby’s blood mixes with your blood, your body may create antibodies to the Rh antigens in the baby’s blood. 

Not to worry mama. Thanks to modern medicine (yay!), you’ll undergo many tests to arm you with all the knowledge for your doctors to act accordingly. In addition to a basic blood test, an antibody screen can show if you’re RH negative and have developed antibodies to your RH positive babe. If so, an injection of Rh immunoglobulin (RhIg), a blood product can prevent this kind of reaction.