There are four main blood groups: A, B, AB and O. People are also either Rhesus D (Rh D) positive or Rhesus D (Rh D) negative.
If you are Rh D negative, your red blood cells do not have the Rh D antigen. Antigens are markers that your body may see as foreign and react to.
Blood type is passed down from your parents. This means that if you are Rh D negative, you could be pregnant with a baby who is Rh D positive. During some events in pregnancy, a small amount of the baby’s blood can mix with yours, which may cause your body to react.