ABSTRACT
Placentation in multiple pregnancies is influenced not only by the number of zygotes but also by the number of blastocysts and, before or after implantation, the division of the inner cell mass. If, in the past, the relationship between the number of the zygotes and their divisions and the gross morphology of the placenta was not always clear, this no longer need be the case. With a minimum of instruction, anyone investigating the placenta of a multiple pregnancy should be able to grasp the link between this structure and the origin of the pregnancy.
