The present invention provides a method for determining the amino acid polymorphisms of heavy gamma chain of immunoglobulins G by a proteomic approach. This method distinguishes the immunoglobulins of the mother and those of the newborn in a blood sample obtained during the first months of the child's life. The invention also relates to the use of this method in the early diagnosis of vertically transmitted diseases in the neonate. The invention also provides peptides distinctive of G3m and IGHG3 alleles, and a kit comprising said peptides.