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Aim: Abnormal fertilization (1PN/3PN) and its accompanying polar body (PB) conditions have been less discussed in poor ovarian responders. By observing the PBs, we analyzed the mechanisms of abnormal fertilization and aimed to explore the role of intracytoplasmic sperm injection/in vitro fertilization (ICSI/IVF) in POSEIDON group 4 patients. Methods: An observational study. All fresh IVF/ICSI cycles from January 2018 to December 2019 were evaluated. The inclusion criteria were POSEIDON group 4. Fertilization and PB conditions were assessed 16–18 h post‐insemination. Primary observation endpoints including normal fertilization, abnormal fertilization, and total fertilization failure rate. Results: A total of 351 cycles involving 180 patients met the inclusion criteria. Of these, 15 cycles reported no retrieved oocytes. Finally, 336 cycles (IVF, n = 267; ICSI, n = 69) were included. A total of 1005 oocytes and 939 embryos were assessed. The mean female age was 40.8 years, and the mean AMH level was 0.6 ng/mL. The normal fertilization rate was 69.7%. The zygote distribution was 18.7% 0PN, 3.9% 1PN, 66.9% 2PN, 9.5% 3PN, and 1.0% ≥4PN. For 1PN zygotes, 59% were denoted as 1PN2PB. The mean 3PN rate was 8.9%. Conclusions: In POSEIDON group 4, most of the monopronucleated zygotes were 1PN2PB. Digyny (3PN1PB), due to failure to extrude the second PB, was the major cause of triploidy in which ICSI could not circumvent. The distribution of abnormally fertilized zygotes was similar in IVF and ICSI. To investigate the mechanisms of abnormal fertilization and assess whether ICSI is necessary, analysis of PB will provide important clues. [ABSTRACT FROM AUTHOR]