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BACKGROUND Whether extended culture allowing selection of embryos with high development potential has any advantage over cleavage-stage embryo transfer remains a matter of debate. Among the currently unsolved questions, the cumulative delivery rate resulting from fresh and frozen embryo transfers needs to be taken into account in both strategies. The aim of our study was, therefore, to compare the efficacy of single embryo transfer either on Day 2 or on Day 5/6 combining fresh and frozen embryo transfers. METHODS A prospective study including 478 couples assigned on a voluntary basis to undergo elective single embryo transfer (eSET, n = 243) on Day 2 or single blastocyst transfer (SBT, n = 235) on Day 5/6 was performed. The primary outcome measurement was the cumulative delivery rate including fresh and frozen–thawed cycles in both groups. RESULTS The delivery rate per cycle following fresh embryo transfer was significantly higher in the SBT group compared with the eSET group (P CONCLUSIONS The observed cumulative delivery rates in this study do not allow us to take a position in favor of SBT or eSET. An improvement in blastocyst cryopreservation may change this attitude. [ABSTRACT FROM AUTHOR]