Splitting Eggs and Sharing Sacs
Multiples are either monozygotic (formed from a single egg and sperm) or dizygotic (formed from two separate eggs and sperm). Monozygotic multiples, more commonly known as identicals, can be further classified by the way they share their common space and maternal resources.
Identical (or monozygotic) twins form when a single fertilized egg (zygote) splits in two. Because they are cut from the same piece of genetic fabric, they are always the same gender. Monozygotic twins also share a placenta. Depending on when the zygote splits, they may share an amniotic sac and/or the chorionic membrane, or they may each have their own personal space and membranes. Twins that split more than a week after fertilization will probably share both the amnion and the chorion, while twins that split early on are more likely to have separate quarters.
Monozygotic (or identical) twins may each have its own set of fetal membranes, or they may share an amniotic sac (monoamniotic), a chorion membrane (monochorionic), or both.
A single zygote that produces twins usually splits within days of fertilization but may wait up to two weeks. Zygotes that split beyond fourteen days increase the risk of resulting in conjoined twins, or twins who haven't separated completely and develop with shared body parts or organ systems. Fraternal twins, because they start life as two eggs fertilized by two sperms, do not run the risk of conjoinment.
Fraternal (or dizygotic) twins are basically siblings in the womb. Each one is created from a separate egg fertilized with its own sperm. Fraternal twins are three times more common than identical twins. They have their own placentas, can be the same or different genders, and may not look any more alike than siblings born individually.