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.
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