Clinical department of Pathology and Cytology “Ljudevit Jurak”,
Clinical Hospital Center “Sisters of Mercy“ Zagreb
Department of Pathology School of Medicine, University of Zagreb
Key words: placenta, twins
SUMMARY. Possible mechanisms of placental development in twins which can in humans can be dyzygotic and monozygotic. Special emphasis is given to the possible complications ow twin pregnancies with regard to the type of placentation. In twin pregnancies with two placentas (biamniotic bichorionic, either separate or fused), the complications are basically the same as in singleton pregnancies, with increased incidence of velamentous insertion of one or both umbilical cords, and increased incidence of single umbilical artery, usually in only one twin. In monozygous twins whose placenta is monoamniotic monochorionic, the main complication is intrauterine death of twins, that usually occurs in the second trimester and is caused by umbilical cords twisting or knots. In monozygous twins that share one chorionic plate, but have two separate amniotic cavities, the greatest danger is the development of feto-fetal transfusion syndrome (twin twin transfusion syndrome-TTS). This syndrome develops because of blood vessel anastomoses between the twins. Pathological and histopathological examination of twin placenta is described, as well as the methodology of assessement of blood vessel anastomoses. Pathological findings in case of intrauterine death of one or both twins with TTTS are also described. The most bizarre complication of biamniotic monochorionic pregnancy is the development of twin reversed arterial perfusion (TRAP). The proposed ways of its development and morphological findings are also described.