Monozygotic twinning in the IVF era: is it time to change existing concepts?

Excerpt

Monozygotic twinning (MZT) is a rare phenomenon among humans. Its incidence after natural conception is about 0.4% of births [1, 2]. However, there seems to be an increased rate of MZT following assisted reproduction technology procedures. In 1984 [3], the first case of MZT associated with IVF was reported. Soon after, Edwards and colleagues called attention to the fact that the conditions of embryonic growth in vitro might influence the incidence of identical twinning [4]. However, this opinion was not in agreement with the observation that artificial induction of ovulation, per se, without IVF treatment, increased the MZT rate [1]. The initial observations on a series of cases of monozygosity in IVF cycles led authors to suggest a link between the physical condition of the zona pellucida, hatching, and the generation of identical twins [5]. The advent of prolonged culture conditions [6] allowed the evaluation of embryo development and the selection of the “best” embryo for transfer at the blastocyst stage. This approach should decrease the number of multiple gestations resulting from the transfer of multiple cleavage stage embryos. However, the emergence of an increased rate of monozygosity after blastocyst transfers was soon associated with the extended culture protocols [7]. Several other publications followed [2, 8‐20]. The reported rate of MZT associated with IVF treatment and blastocyst transfer can be as high as 5 and 6% [14]. Our group experienced the case of a quintuplet gestation, after the transfer of two blastocysts [19]. …