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. Potencial de desarrollo de los embriones de desarrollo lento: mórulas en día 5 comparadas con mórulasen cavitación en día 5. Describir y comparar las tasas de embarazo evolutivo entre las mórulas y las mórulas en cavitación (CAVM) transferidas en día 5, describir y comparar la tasa de blastulación entre las mórulas en día 5 y CAVM y describir la tasa de embarazo de estos blastocistos de desarrollo lento en un ciclo de transferencia de embriones congelados (FET). Estudio retrospectivo de cohortes. Un único centro médico de tercer nivel. Embriones de desarrollo lento: 3.321 ciclos que incluyeron 10.304 embriones en día 5 que fueron cultivados hasta día 6. Desarrollo de las mórulas y CAVM a estadio de blastocisto. Tasa de blastulación. Los ciclos de transferencia en fresco incluyeron a 186 pacientes con 82 embriones en estadio de mórula y 104 embriones en estadio de CAVM. Las tasas de embarazo (15,8% vs 21,1%) y las tasas de embarazo evolutivo (15,8% vs 17,3%) fueron comparables entre los grupos. El grupo de estudio incluyó 10.304 embrion