Monozygotic pregnancies from transfers of zona-free blastocysts
Fertility and Sterility
To minimize the risk of identical twinning the zona pellucida (ZP) was completely removed before blastocyst ET in all blastocyst transfers performed between May 1, 1999 and April 30, 2000. Compared to prior, zona intact, blastocyst transfers carried over between September 1, 1998 and April 30, 1999 no significant difference was noted regarding implantation, pregnancy, monozygotic pregnancy, or multiple gestation rates, thereby indicating that monozygosity can arise independently of the ZP. © 2004 by American Society for Reproductive Medicine.
Frankfurter, D., Trimarchi, J., Hackett, R., Meng, L., & Keefe, D. (2004). Monozygotic pregnancies from transfers of zona-free blastocysts. Fertility and Sterility, 82 (2). http://dx.doi.org/10.1016/j.fertnstert.2004.02.105