Fraternal twins, also known as dizygotic twins since they come from two eggs and two sperm, always have their own placenta and surrounding sacs. About 70% of monozygotic twins, or single egg and single sperm made identical twins, will end up sharing a single placenta which can cause potential complications. The most problematic is also the least likely (about 1%) and affects only identical twins, is the combination of a single placenta and a single sac.
Zygosity refers to how many eggs were involved with the conception of your twins. One egg (ova) makes monozygotic or identical twins, but two eggs are a set of dizygotic or fraternal twins. Chorionicity talks about how the membranes are laid out.
