Received August 19, 2024
Accepted June 02, 2025

Objective: The aim of the study was to analyze the effectiveness of the day 3 ET strategy, and the morphology of the transferred embryos, in patients from POSEIDON and non-POSEIDON groups. Methods: 600 cycles of patients meeting the POSEIDON criteria and 600 non-POSEIDON cycles nonwere analyzed to determine the proportion of cycles with an ET on days 3 or 5, or FET. Then we reviewed 330 day 3 ETs to compared the developmental stage, morphology, zona pellucida thickness and implantation potential of embryos transferred on day 3 from POSEIDON and non-POSEIDON patients. Results: Most cycles of POSEIDON patients end with ET on day 3 (42%) or without transfer (37%). In contrast, most cycle of non-POSEIDON patients end with FET (44%) and just 9% is canceled. The lowest percentage of embryos at the morula stage was recorded in POSEIDON groups III (10%) and IV (9%). The average number of cells in embryos was comparable in all groups. The largest percentage of top-quality embryos (grade A) were in POSEIDON group I (47%) .The highest implantation potential were observed in the non-POSEIDON group <35Y (28%), and in POSEIDON groups I (28%) and III (26%). The highest incidence of miscarriage was recorded in all POSEIDON and non-POSEIDON groups that included patients who were ≥35 years of age. Conclusion: The day 3 ET strategy still seems optimal for POSEIDON patients. The prognosis depends on which Poseidon group the patient is in. The best prognosis is for group I and the worst for group IV.