What characteristic defines a monochorionic-diamniotic twin pregnancy?

In a monochorionic-diamniotic twin pregnancy, the defining characteristic is that the twins share a single placenta (monochorionic) while being housed in separate amniotic sacs (diamniotic). This arrangement typically occurs when the embryos split at an early stage of development, allowing them to share placental blood supply and nutrients while still having individual amniotic environments.

This situation is crucial because it significantly impacts the management and monitoring of the pregnancy, given the increased risks associated with shared placentation such as twin-to-twin transfusion syndrome. The condition allows for distinct amniotic sacs, which can provide some separation and reduce the risks associated with sharing a single sac, such as entanglement of the umbilical cords.

The other choices do not accurately describe the characteristics of a monochorionic-diamniotic pregnancy and reflect different types of twin pregnancies: two placentas and two sacs refer to dichorionic-diamniotic twins, while twin fusion within a single sac is not a standard classification for any common type of twin pregnancy.