Twin Sisters

When the division of the developing zygote into 2 embryos occurs, 99% of the time it is within 8 days of fertilization
Sometimes, monochorionic twins also share the same amnion. This situation occurs in 1–2% of monozygotic twin pregnancies.
Monoamniotic twins are always monozygotic (identical twins).
The survival rate for monoamniotic twins is somewhere between 50% to 60%.