Monochorionic/diamniotic twins are always:

Monochorionic/diamniotic twins originate from a single zygote that undergoes division, leading to the formation of two embryos. This process results in identical twins, as they share the same genetic material. The term "monochorionic" indicates that there is one chorion—the outer membrane that surrounds the embryo and forms part of the placenta—while "diamniotic" refers to each twin being in its own individual amniotic sac.

Since these twins develop from one fertilized egg, they are classified as monozygotic. This contrasts with dizygotic twins, which result from the fertilization of two separate eggs by two separate sperm, leading to the formation of two genetically distinct individuals, each with its own chorion and amniotic sac.

In the context of sharing a placenta, the statement that monochorionic twins share a placenta is accurate, but this sharing is not explicitly what counts them as monozygotic. The defining feature of monozygotic twins is their origin from a single fertilization event, rather than the presence of a single placenta.