ABSTRACT

Although the mechanisms by which MZ twins are formed in the human are not completely understood, many investigators postulate that the splitting of the inner cell mass at an early stage of development may cause duplication of the embryo1. Splitting of the zygote may occur at any time during the first 2 weeks after fertilization, resulting in various forms of MZ twinning (see Chapter 24). Zygotic splitting probably begins with the protrusion of some trophectoderm cells through a small opening in the zona pellucida. Factors that appear to predispose to splitting include alterations in thickness of

the zona pellucida, time of implantation and assisted reproductive technologies (ART).