Background Compaction is considered an important outward marker of the embryonic genome activation (EGA), the embryonic metabolic shift and the starting point of differentiation and cell fate determination. Thus, although the morula is one of the most understudied stages, it represents a critical transition point which can provide valuable information about the embryo development and potential.

Objectives Look into the causes and mechanisms driving partial compaction and asses its potential as a predictive tool for embryo selection. The main hypothesis was that partial compaction is a negative marker and can assist in predicting lower embryo quality, potential, and reproductive outcomes.

Methods 405 embryos derived from 100 patients undergoing 114 IVF cycles from January to July 2022 in Hospital Ruber Internacional were analysed retrospectively through Time Lapse Technology.  The parameters considered were the maternal age, the cleavage pattern, the number of blastomeres prior to compaction, the type of compaction, the number of excluded or/and extruded cells, the decompaction and recompaction events, the ICM and TE grade, the tSB (start of blastulation) and tB (blastulation completed) timings, the ploidy status, the pregnancy rates and the live birth rates. Fisher’s exact test and Pearson’s chi squared test were used as the main statistical tests to assess if differences or associations were statistically significant.

Results Embryos exhibiting partial compaction were more frequent in patients over 37 y.o. (p=0.036) and in embryos coming from abnormal cleavage episodes (p<0.001). A strong association was identified between FC and higher blastocyst rates (p < 0.0001), faster tSB and tB timings and better ICM and TE grades (p<0.001 in all cases). Any significant differences were observed when assessing ploidy. A tendence was observed in full compaction patterns achieving more pregnancy and live birth rates, but the difference was not statistically significant (p>0.05).

Conclusion Partial compaction patterns were identified as negative indicators of blastocyst yield, embryo morphokinetics and morphological grade. Further studies are needed to assess if the difference in ploidy and live birth rates is statistically significant and if the incipient perspective of partial compaction as a self-repair mechanism is certain and of utility.