Estimation of iatrogenic monozygotic twinning rate following assisted reproduction: Pitfalls and caveats.

The true incidence of monozygosity after assisted reproduction is unknown. Proxy estimations, such as counting the number of monochorionic twins, using Weinberg's differential rule, and counting cases where the number of fetuses exceeds the number of transferred embryos, are less accurate than zygosity assessment in all twins. These methods commonly underestimate the true frequency of zygotic splitting because they disregard like-sexed monozygotic-dichorionic twins, consider twin births rather than twin pregnancies, and do not count the number of higher-order multiples with a monozygotic pair. Because zygotic splitting following assisted reproduction is of biological interest as well as of clinical significance, efforts should be directed to increase the accuracy of zygosity determination. This will improve understanding of the zygotic splitting phenomenon and its relation to pathologic processes.