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Objective: To compare live-birth rates, blastocyst to live-birth efficiency, gestational age, and birth weights in a large cohort of patients undergoing single versus double thawed blastocyst transfer., Design: Retrospective cohort study., Setting: Assisted reproduction technology (ART) practice., Patient(s): All autologous frozen blastocyst transfers (FBT) of one or two vitrified-warmed blastocysts from January 2009 through April 2012., Intervention(s): Single or double FBT., Main Outcome Measure(s): Live birth, blastocyst to live-birth efficiency, preterm birth, low birth weight., Result(s): Only supernumerary blastocysts with good morphology (grade BB or better) were vitrified, and 1,696 FBTs were analyzed. No differences were observed in patient age, rate of embryo progression, or postthaw blastomere survival. Double FBT yielded a higher live birth per transfer, but 33% of births from double FBT were twins versus only 0.6% of single FBT. Double FBT was associated with statistically significant increases in preterm birth and low birth weight, the latter of which was statistically significant even when the analysis was limited to singletons. Of the blastocysts transferred via single FBT, 38% resulted in a liveborn child versus only 34% with double FBT. This suggests that two single FBTs would result in more liveborn children with significantly fewer preterm births when compared with double FBT., Conclusion(s): Single FBT greatly decreased multiple and preterm birth risk while providing excellent live-birth rates. Patients should be counseled that a greater overall number of live born children per couple can be expected when thawed blastocysts are transferred one at a time., (Published by Elsevier Inc.)