What are monochorionic twins?

There are two types of twins: fraternal (dizygotic) and identical (monozygotic) twins. Fraternal twins occur when two different eggs are independantly fertilized by two different sperm cells. Therefore, fraternal twins have different genes and generallhy do not look alike. Fraternal twins are essentially two ordinary siblings who happen to be born at the same time.  Fraternal twins can be boy-boy, girl-boy, and girl-girl twins.

Identical twins occur when a single egg is fertilized by one sperm cell, whereafter it splits into two seperate embryos. Since these embryos are derived from the same egg and sperm cell, they share the same genes and look identical. Identical twins are always pairs of the same sex (girl-girl, boy-boy).  

Alongside to the distinction between mono and dyzygotic twins, twins are also classified based on number of placentas and amniotic sacs that they have. 

When the babies each have their own placenta, it's called a dichorionic twin pregnancy (di = 2, chorion = placenta), when the babies share a single placenta, they're called monochorionic twins (mono = 1, chorion = placenta) genoemd. If both babies have their own amniotic sac, it's referred to as a diamniotic genoemd. When both babies share a single amniotic sac, they're named monoamniotic twins.

Fraternal twins always have two placentas and two amniotic sacs. In identical twins, it is more complex. In identical twins, the moment of embryo division determines the number of placentas and amniotic sacs. When the division occurs within 3 days after fertilization, each baby will each have their own placenta and amniotic sac, just like in fraternal twin pregnancies. If the division of the embryo occurs between 4-8 days, the identical twin will share 1 placenta, but will still have 2 amniotic sacs. When the division occurs even later, between 9-12 days after fertilization, the babies will share both their placenta and amniotic sac.  The figure below present a an overview of the different types of twins.