Eighteen [human] cases of trisomies 13, 18 or 21 were diagnosed by examining the karyotype or on clinical criteria. When the placenta was examined in 60% of cases, there was a marked retardation of villous maturation with frequent persistence of embryonic forms of villi. Failure of the villi to mature was often a reflection of pathology in the oocyte which started to show itself at the beginning of the pregnancy.