Context: The process of hatching in mammalian embryo, involving the escape of the blastocyst from the zona pellucida (ZP) and was first observed by Lewis and Gregory in 1929. Objective: This study investigated the correlation between blastocyst collapse and pregnancy/implantation rates following elective single transfer at blastocyst stage (eSET). Methods: This study was carried out between January2016-August2017 at EFREC,RIE-Edinburgh. Following transfer, each blastocyst was retrospectively allocated to one of two groups (collapsed or not collapsed). Pregnancy and implantation rates were analysed. Patient(s): 197 couples having an eSET on day5 were included in the study. Embryos were cultured individually in an EmbryoSlide™, using an EmbryoScope™ imaging system in 6% CO2, 5% O2, 89% N2, using single step medium (G-TL™ Vitrolife). Intervention(s): Morphological assessment was made by examining a video of development using the associated EmbryoViewer software. Main Outcome Measure(s): Blastocyst were analysed by measuring the maximum volume reduction during development and defined as having collapsed if there was ≥50% of the surface of the TE was separated from the ZP. Result(s): A total of 197 cycles were analyzed. 16 blastocysts collapsed once or more (8.1%), the remaining 181 either contracted minimally or neither contracted during development (91.9%%). A significantly higher pregnancy rate (BhCG>5) of 72% (142/197) was observed when blastocysts which had not collapsed were replaced compared to cycles in which collapsed blastocysts were replaced (pregnancy rate 38.9%; 14/36). This trend was repeated when considering the implantation and clinical pregnancy rates of both groups 62.4% (123/197) and 56.3% (111/197) in the no collapse group compared to 19.4% (7/36) and 16.6% (6/36) in the collapsed group. Conclusions: The pattern of blastocyst collapse could improve IVF/ICSI outcome following eSET at blastocyst stage, therefore we suggest to include as a negative marker for embryo selection.