Monozygotic twinning is not increased after single blastocyst transfer compared with single cleavage-stage embryo transfer.

OBJECTIVE: To compare the incidence of monozygotic twinning between cleavage-stage and blastocyst-stage embryo transfer in a large cohort of patients undergoing single embryo transfer.

DESIGN: Retrospective study.

SETTING: Dutch-speaking Free University of Brussels.

PATIENT(S): This study covered the period between July 2003 and December 2005. 1,951 fresh IVF/ICSI cycles in which single embryo transfer was performed were retrospectively reviewed. Only the first cycle of each patient was included.

INTERVENTION(S): Five hundred eighty seven (n = 587) cycles that resulted in clinical pregnancies were identified; 308 after single day-3 embryo transfer and 271 after single blastocyst transfer.

MAIN OUTCOME MEASURE(S): The incidence of monozygotic twinning.

RESULT(S): Overall, 13 cases (2.2%) of monozygotic twinning were observed, 2.6% in the cleavage-stage group (n = 8/308) and 1.8% in the blastocyst group (n = 5/271). No statistically significant differences were observed in the probability of monozygotic twinning between the Cleavage-stage and the Blastocyst group (difference: +0.8%; 95% CI, -1.97 to +3.41). All of these pregnancies resulted in the delivery of 24 healthy babies. The crude odds ratio for the incidence of monozygotic twinning after day-5 embryo transfer was calculated to be 0.71 (95% CI, 0.23-2.18).

CONCLUSION(S): To investigate the potential association between the day of embryo transfer (day 3 or 5) and the incidence of monozygotic twinning, the clinical pregnancies analyzed should have been established after single embryo transfer. The current study represents the first methodologically appropriate study attempting to investigate the above research question. Our findings support that opting for blastocyst transfer does not increase the probability for monozygotic twins.