Twin Sisters

It is a common misconception that two placentas means twins are dizygotic (non-identical). But if monozygotic twins separate early enough, the arrangement of sacs and placentas in utero is indistinguishable from dizygotic twins.
• Dichorionic-Diamniotic
Normally, twins have two separate (di- being a numerical prefix for two) chorions and amniotic sacs, termed Dichorionic-Diamniotic or "DiDi". It occurs in almost all cases of dizygotic twins (except in very rare cases of fusion between their blastocysts ), in 99.7% of all pregnancies, and in 18–36% (or around 25%) of monozygotic (identical) twins.
DiDi twins have the lowest mortality risk at about 9 percent, although that is still significantly higher than that of singletons.