A pregnant woman can make IgG blood group antibodies if her fetus has a blood group antigen that she does not have. This can happen if some of the fetus' blood cells pass into the mother's blood circulation (e.g. a small fetomaternal hemorrhage at the time of childbirth or obstetric intervention), or sometimes after a therapeutic blood transfusion. This can cause Rh disease or other forms of hemolytic disease of the newborn (HDN) in the current pregnancy and/or subsequent pregnancies. If a pregnant woman is known to have anti-RhD antibodies, the RhD blood type of a fetus can be tested by analysis of fetal DNA in maternal plasma to assess the risk to the fetus of Rh disease.[36] One of the major advances of twentieth century medicine was to prevent this disease by stopping the formation of Anti-RhD antibodies by RhD negative mothers with an injectable medication called Rho(D) immune globulin.[37][38] Antibodies associated with some blood groups can cause severe HDN, others can only cause mild HDN and others are not known to cause HDN.[3]