ABSTRACT

Twins are currently classified into two main types, monozygotic (MZ) and dizygotic (DZ). A more complex classification was proposed some years ago by Keith and Oleszczuk to recognize some of the problems that may attach to the simplistic MZ-DZ paradigm (see below for details). DZ twins occur when two oocytes are fertilized by two spermatozoon. MZ twins occur when one oocyte is fertilized by one spermatozoon and at some later time divides to form two embryos. The causes of this split are uncertain. Familial MZ twinning is rare, but does occur. Similarly, many families appear to be ‘twin’ prone, but controversy exists on the exact cause of this phenomenon. MZ twins occur at approximately two to three times the normal rate in pregnancies resulting from assisted reproductive technologies (ART) , be they in vitro fertilization (IVF) or ovulation induction.