Risk of monozygotic twinning with blastocyst transfer decreases over time: an 8-year experience.

OBJECTIVE: The purpose of our study is to compare the occurrence of monozygotic twinning (MZT) from blastocyst transfer (BT) in our program between an earlier and more recent time period.

DESIGN: Retrospective.

SETTING: Academic IVF practice.

PATIENT(S): All pregnancies conceived between March 2002 and December 2005 (N = 932) in our program were compared to pregnancies conceived before March 2002 (N = 554), which were the subject of a previous study.

INTERVENTION(S): None.

MAIN OUTCOME MEASURE(S): The incidence of MZT with day 3 embryo transfer and BT were compared between the study and control groups.

RESULT(S): During the study period, the rate of MZT was not significantly different for BT at 2.3% (9/385) compared to day 3 embryo transfer at 1.8% (10/547). This rate of 2.3% for BT was significantly lower than the rate of 5.6% (11/197) reported at our institution for BT before March 2002.

CONCLUSION(S): Our study suggests that the risk of MZT with BT is significantly lower in the more recent time period and is in the range of what is seen with cleavage stage transfer. It is likely that improvements in culture systems as experience is gained with BT played a role.