Blastocyst transfer is not associated with increased rates of monozygotic twins when controlling for embryo cohort quality

Objective: To compare monozygotic twinning (MZT) rates in patients undergoing blastocyst or cleavage-stage ET.

Design: Retrospective cohort.

Setting: Academic research center.

Patient(s): Autologous, fresh IVF cycles resulting in a clinical pregnancy from 1999 to 2014.

Intervention(s): None.

Main outcome measure(s): Monozygotic twin pregnancy in blastocyst-stage transfer vs. cleavage-stage transfer when controlling for patient prognosis and embryo cohort quality factors.

Result(s): There were a total of 9,969 fresh transfer cycles resulting in a pregnancy during the study period. Of these pregnancies, 234 monozygotic twin pregnancies were identified (2.4%). Of all transfers, 5,191 were cleavage-stage and 4,778 were blastocyst-stage transfers. There were a total of 99 MZT identified in the cleavage-stage group (1.9%) and 135 MZT in the blastocyst ET group (2.4%), which was significant. Multivariable logistic regression revealed that increasing age was associated with a significant reduction in MZT, regardless of transfer order. Embryo cohort quality factors, including the number and proportion of six- to eight-cell embryos and availability of supernumerary embryos, were also significant. When controlling for patient age, time period during which the cycle took place, the number and proportion of six- to eight-cell embryos, and availability of supernumerary embryos, there was no longer a difference in MZT rate between blastocyst and cleavage transfer.

Conclusion(s): Patient prognosis and embryo cohort quality seem to be major factors in MZT rate in women undergoing blastocyst transfer. Although technology-based effects cannot be excluded, patient and embryo characteristics play an important role.