What is the most common type of monozygotic twins?

The most common type of monozygotic twins is mono-di, or monochorionic-diamniotic twins. This occurs when a single fertilized egg (zygote) splits into two embryos within the first few days after fertilization. As a result, these twins share a placenta (monochorionic) but have separate amniotic sacs (diamniotic).

This type is the most prevalent because most monozygotic twinning happens during the early stages of development, specifically within the first three days post-fertilization when the embryo is yet to implant. The division after implantation leads to the dichorionic (two chorions, which would typically be the case for dizygotic twins) or monoamniotic (both twins share the same amniotic sac) scenarios, which are less common.

The presence of a single placenta can pose certain risks to the twins, such as twin-to-twin transfusion syndrome, a condition where blood flow is unequally distributed between the twins, leading to complications. Understanding the type of twinning is crucial for monitoring and managing pregnancies effectively.