Twin Sisters

Monochorionic twins share the same placenta.
Monochorionic twins generally have two amniotic sacs (called Monochorionic-Diamniotic "MoDi"), which occurs in 60–70% of the pregnancies with monozygotic twins. Monochorionic-Diamniotic twins are almost always monozygotic, with a few exceptions where the blastocysts have fused.
This occurs during days 4-8.