Samples from infants less than 4 months old will not regularly contain expected Anti-A or Anti-B in serum; therefore, only RBC ABO typing is required. Rh-positive and Rh-negative phenotypes are determined by the presence or absence of D antigens on RBCs. When Rh typing is performed for newborns of Rh-negative mothers (to determine maternal RhIG candidacy) the method for Rh typing must be capable of detecting weak expression of D; therefore, Weak D testing may also be performed. When fetomaternal ABO incompatibility exists, potential for ABO Hemolytic Disease of the Fetus and Newborn (HDFN) exists.