ABSTRACT
The principles addressed in the preceding paragraph pertain to triplet and higher-order placentation as well. With regard to triplets, a fused placental mass is more common, regardless of zygosity, simply because of uterine crowding. Figure 24.2 shows the mechanisms of MZ, DZ and trizygotic (TZ) triplets with different types of membranes. It is readily apparent that examination of the membranes alone cannot establish zygosity if the placental mass is trichorionic. Similarly, if the placental mass is dichorionic, the zygosity may be MZ or DZ. Finally, only when the (single) placenta is monochorionic can one truly establish monozygosity, regardless of the number of fetuses. Thus, the principle stated in the preceding paragraph, i.e. that placental examination by itself is insufficient to establish zygosity in all cases, can be extended to the membranes as well. The general rule in triplets is to have a single placental mass, and a single placental mass can result from MZ, DZ or TZ origins. Table 24.2 outlines placental structure in triplets and complements data presented in Figure 24.2.
