The pathological changes in these structures have been very imperfectly investigated, and there are many of the lesions whose nature is not clearly understood.

The decidua is formed, under the influence of pregnancy, from the mucous membrane of the uterus. This structure is greatly thickened and otherwise altered in structure in order to accommodate the foetus. The membranes formed by the foetus are the chorion and the amnion. The former is originally covered by villi, which, in a limited area, go to form the foetal part of the placenta. The placenta is a joint product of the maternal and foetal structures, the decidua furnishing the maternal and the chorion the foetal portion.