Fetal/neonatal alloimmune thrombocytopenia (FNAIT) is defined as a platelet count <150,000/μL due to reaction between maternal antibodies and antigens located on thrombocytes of the fetus/neonate. Such a kind of pathology occurs when a mother does not possess specific human platelet antigens (HPA), which are inherited as an infant from the father. HPA-1a is an antigen that most often causes FNAIT in the Caucasian race. Frequency of FNAIT has been estimated as 1:350–1:5000. In the pathogenesis of FNAIT, the mother organism is immunized and produces alloantibodies from which IgG pass through the placenta, enter fetal circulatory system and cause platelet destruction. It usually takes place at the end of the second trimester. FNAIT can occur in the first pregnancy; nonetheless, it is more probable and connected with higher severity in subsequent gestations. It is caused by the fact that immunization usually takes place during the first labour, which enables production of alloantibodies in the next pregnancies. Nevertheless, other ways of immunization are also possible, which ensures that FNAIT cases in the first pregnancy are not casuistic and occur more often than RhD hemolytic disease of the newborn. Thrombocytopenia leads to coagulation disorders, and consequently to bleedings. FNAIT can be manifested not only by some petechiae on the skin, but also by severe hemorrhages in the body cavities and gastrointestinal tract or by intracranial hemorrhages (ICH). In case the symptoms are presented by an infant, platelet transfusions are performed, and IVIG (intravenous immunoglobulin) therapy is administered.