The incidence of monozygotic twinning in assisted reproductive technology: analysis based on results from the 2010 Japanese ART national registry.

Purpose: To assess the incidence of monozygotic twinning (MZT) among cases undergoing assisted reproductive technology (ART) treatment.
Methods: We performed a retrospective observational study and analyzed the data of patients who were registered in the national ART registry system of Japan from January to December 2010; only the data of patients with single embryo transfer (ET) were included.
Results: Of 30,405 pregnancies, 425 resulted in MZT following fresh and frozenthawed ET. The MZT incidence among women undergoing ART was 1.4 %. Multiple logistic regression analysis indicated that cases undergoing fresh and frozen-thawed ET, blastocyst transfer had a significantly increased MZT rate (P < 0.01). Assisted hatching (AH) and frozen-thawed ET and maternal age did not significantly affect the MZT incidence. Of 8510 fresh ET pregnancies, 104 resulted in MZT. Multiple logistic regression analysis indicated that blastocyst transfer significantly increased the MZT rate in cases undergoing fresh ET. Ovarian stimulation, intracytoplasmic sperm injection, AH, and maternal age did not significantly affect the MZT incidence.
Conclusions: Blastocyst transfer was associated with an increased MZT incidence. We have to be aware of the potential risk of MZT caused by blastocyst transfer. However, further studies are required to assess the correlation among specific AH types, embryo culture conditions, and MZT incidence.