ABSTRACT
The common denominator of all assisted reproductive technologies (ART) is ovarian (hyper)stimulation, resulting in an excess of ripe ovarian follicles, and leading to multiple oocytes available for fertilization. Multiples conceived with ART are therefore likely to be polyzygotic. However, as early as the late 1980s, and almost simultaneously, two reports claimed the occurrence of monozygosity among ART twins1,2. Whereas the first was a hospital-based case series related to in vitro fertilization (IVF)1, the other2 was based on data from the East Flanders Prospective Twin Study (see Chapter 6). The latter dataset enabled Derom and colleagues2 to deduce that monozygosity occurred significantly more frequently among pregnancies after artificial induction of ovulation (1.2%), compared with the expected frequency (0.45%) among spontaneous twins and triplets. These findings are of importance because no other biologic mechanism influencing the monozygotic (MZ) twinning rate has ever been identified (see Chapter 28). In this volume, two separate discussions are devoted to the phenomenon of MZ division following ART (see Chapters 28 and 29). This chapter, however, highlights various unanswered questions related to this phenomenon.
