Incidence of monozygotic twins in blastocyst and cleavage stage assisted reproductive technology cycles

Objective: To study the incidence of monozygotic twins (MZT) in blastocyst and cleavage stage ET. Design: Retrospective review. Patient(s): Four hundred ninety-six women undergoing IVF/intracytoplasmic sperm injection cycles at a private assisted reproductive technology (ART) center. Intervention(s): Patients undergoing ART were divided according to the stage of ET into blastocyst transfer (BT) and cleavage stage (CS) ET. Main Outcome Measure(s): Incidence of MZT as noted on vaginal ultrasound at 6 to 8 weeks. Result(s): There were 374 (75.4%) BT cycles, and 122 (24.6%) CS cycles. Women in the BT group were significantly younger, had more oocytes retrieved, and had less embryos transferred compared with the CS group. The clinical pregnancy rate was significantly higher in the BT group at 67.9% (254 of 374). compared with 37.7% (46 of 122) among the CS group. There was a significantly higher incidence of multiple gestation in the BT group compared with CS group (37.4% compared with 19.6%). The overall incidence of MZT was 1.3%, but differed with the stage of ET: the incidence of MZT was 1.57% (4 of 254) in the BT group, and 0 (0 of 46) in the CS group. Conclusion(s): Contrary to the older published literature on MZT in BT cycles, the incidence of MZT is low. Women undergoing ART therefore should not be discouraged to undergo BT for fear of MZT, especially in light of the higher pregnancy rate and lower number of transferred embryos noted in those cycles compared with cleavage stage transfers. (Fertil Steril(R) 2010;93:642-5. (C)2010 by American Society for Reproductive Medicine.)