2 babies 2 sacs MC
Some monozygotic (mz, identical) pregnancies have 1 placenta
mass but 2 sacs and are called monochorionic diamniotic.
There is a risk of twin-twin transfusion which is a serious problem for both babies. Less worrisome is when it appears to be 1 but really 2 placentas are adjacent or "fuse"; there are 2 if boy+girl seen on ultrasound or tests after delivery show dz twins or dichorionic mz placenta.