What are monochorionic twins?

Monochorionic twins are twins who develop from a single fertilized egg that implants in the uterus and then splits into two separate embryos within the first few days of development.

Monochorionic twins are always identical, meaning they share the same genetic material and have the same physical characteristics. They are also always enclosed within the same amniotic sac, which is the fluid-filled membrane that surrounds and protects the developing fetus during pregnancy.

Approximately one-third of all fraternal twins are monochorionic. The remaining two-thirds of fraternal twins are dichorionic, which means they develop from two separate fertilized eggs that implant in the uterus at the same time.