The eternal dilemma: are embryos better nurtured in utero or in vitro?

One of the most debated topics in reproductive medicine is the optimal timing of embryo transfer. While blastocyst-stage transfer has become widely adopted due to the higher blastocyst implantation potential and possibility of synchronizing the transfer with endometrial receptivity, it may not offer universal benefits. A significant proportion of embryos fail to reach the blastocyst stage in vitro despite being chromosomally normal, raising concerns that extended culture may lead to unnecessary euploid embryo loss. This paper challenges the assumption that in-vitro culture conditions adequately mimic the uterine environment, and proposes the use of a more individualized approach to the timing of embryo transfer. Probability calculations have been developed to estimate an embryo's live birth potential at the cleavage stage, assessing whether earlier transfer improves the likelihood of live birth compared with extended culture. This is particularly critical for poor-prognosis patients (PPP), who typically produce fewer embryos and risk having no viable blastocysts for transfer. Cleavage-stage transfer may provide a valuable alternative for PPP, allowing the uterine environment to support continued development and rescue of embryos that might otherwise become arrested in vitro. These findings emphasize the need for personalized IVF strategies and highlight the clinical relevance and rationale for day 3 transfers in PPP.