Each embryo has a blood type. Your red blood cells carry markers called antigens on their surface that provide evidence of your blood type. For a long time it was thought there were only four blood types: O being the most common, A, B, and AB being the least common. Then that was expanded to eight blood types because of Rh factor and other antigen/antibody markers. (Reportedly there are more than 600 known antigens besides A and B.) The vast majority of blood types fall into one of the major ABO groups. However, for a small percentage of the population, it can be like looking for the proverbial needle in a haystack to find another person with the same blood type, which can be an issue if the individual suddenly requires a blood transfusion. Do you know your blood type? Mine happens to be O.