Triplet pregnancies after single blastocyst embryo transfer-how often and why do these occur?

Single embryo transfer (sET) has been adopted by assisted reproductive technology (ART) clinics globally to increase the rate of singleton births and reduce multi-fetal pregnancies. Independent ART registry-based studies have reported an association between monozygotic (MZ) splitting and sET, especially after single blastocyst embryo transfer (sBET). While MZ twins after sBET are not uncommon, MZ triplet pregnancies after sBET are exceptionally rare. In this context, we report two cases of MZ triplet pregnancies after sBET. The first case is of a 29-year-old woman with a unicornuate uterus who conceived a tri-chorionic triplet pregnancy after single euploid blastocyst transfer in a medicated frozen embryo transfer (FET) cycle. The second case is of a 34-year-old woman with a triplet pregnancy consisting of monochorionic diamniotic twins and a singleton after the transfer of an untested blastocyst in a natural FET cycle. Using these cases, we focus on the epidemiology and putative mechanisms of MZ splitting. Given that sBET does not eliminate the risk of multiple pregnancy entirely, it is important to counsel couples about the risks of MZ twins and triplets with sBET.