Developmental potential of slow-developing embryos: day-5 morulae compared with day-5 cavitating morulae.

To describe and compare the ongoing pregnancy rate between morulae and cavitating morulae (CAVM) transferred on day 5, to describe and compare the blastulation rate between day 5 morulae and CAVM, and to describe the pregnancy rate of these slow-developing blastocysts during a frozen embryo transfer (FET) cycle. Retrospective cohort study. Single tertiary care medical center. Delayed-development embryos: 3,321 cycles that included 10,304 embryos on day 5 that were cultured until day 6. Development of morula and CAVM to the blastocyst stage. Blastulation rate. The fresh embryo transfers comprised 186 patients with 82 embryos at the morula stage and 104 embryos at the CAVM stage. The pregnancy rate (15.8% vs. 21.1%) and the ongoing pregnancy rate (15.8% vs. 17.3%) were comparable between the groups. The study group included 10,304 day-5 delayed embryos: 5,395 morulae and 4,909 CAVM on day 5. The blastulation rate was statistically significantly higher in the CAVM group compared with the morula group (39.2% vs. 20.4%). We included 201 FET cycles: 77 warmed blastocysts that developed from a morula on day 5 and 124 warmed blastocysts that developed from CAVM on day 5. The clinical pregnancy rate was comparable between the two groups per embryo transfer (21.3% vs. 24.7%). Transferring of fresh, slow-developing embryos seems to improve the cycle outcomes compared with culturing for another day and then vitrifying and thawing later.

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