The type of placenta nurturing identical twins plays a significant role in the development of complications in multiple gestation. Identical twins may either have their own separate placentas or they may share a common placenta. The impetus for and the timing of the embryo to split into identical twins is unknown, but the later this occurs the more complications are seen.
The type of placenta is determined by when, in days, the embryo randomly splits into twins following the fertilization of the egg (conception). Twinning within the first four days results in dichorionic or separate placentas similar to those found in fraternal twins (see Figure 1). These identical twins have the lowest complication rates. Twinning four or more days after conception will lead to a shared or monochorionic (MC) placenta. Between four to eight days the MC twins will have separate sacs of water (diamnionic) despite a shared placenta, but after eight days they will also be in the same sac (monoamnionic). Diamnionic monochorionic (4 to 8 day split) twins are the most common placental type for identical twins, and most cases of TTTS occur in this group. It is important to note that if the twins have a MC placenta it is absolute proof that they are 'identical'.
The MC placenta contains two anatomic variables that are thought to develop randomly, which will contribute to and explain why, when and to what degree TTTS will affect the twins. The first is the presence of blood vessels in the placenta that connect the umbilical cords and circulations of the twins, and the second is the variations in the way the twins share their common placenta. These are discussed separately below, but in some MC twins both these abnormalities may be present.