Video - Development of twins

Twins refer to two offspring developed by the same pregnancy. They can either be fraternal or identical twins. Fraternal or dizygotic twins develop when two separate eggs are released at the same time during ovulation; each gets fertilized by its own sperm, growing into a different zygote. On the other hand, identical twins or monozygotic twins develop from a single zygote that splits into two separate embryos into the early stages of development, usually during the first thirteen days of development.

Twins are referred to as monochorionic-monoamniotic (mono-mono) when they share one placenta and one amniotic sac; dichorionic-diamniotic (di-di) when each one has its own placenta and its own amniotic membrane; and monochorionic-diamniotic (mono-di) when they share one placenta but with separate amniotic sacs.