In order to determine the incidence of confined placental mosaicism (C
PM) in term placentae and to show the presence of specific sites and t
he effect on fetal development, 125 placentae from uneventful pregnanc
ies were analysed by cytogenetic methods. The incidence was at least 4
.8 per cent and there were no specific sites on the placenta. Although
the number of cases is still too small, we found CPM to be associated
with intrauterine growth retardation in six cases.