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.


 Retrospective cohort.


 Academic research center.


 Autologous, fresh IVF cycles resulting in a clinical pregnancy from 1999 to 2014.


 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.


 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.
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