Day 1 versus Day 2 Intracytoplasmic Sperm Injection: Comparison of Fertilization and Embryo Development Outcomes in IVF Patients

Background: Intracytoplasmic sperm injection (ICSI) has become an essential assisted reproductive technology for overcoming male-factor infertility and improving fertilization in in vitro fertilization (IVF) cycles. Although ICSI is commonly performed on the day of oocyte retrieval after mature metaphase II (MII) oocytes are identified, immature oocytes may undergo in vitro maturation (IVM) and be injected on the following day. The influence of delayed ICSI on embryological and clinical outcomes remains controversial, with inconsistent findings among previous studies.
Objective: To compare fertilization rate, embryo quality, and pregnancy outcomes between Day 1 and Day 2 ICSI following oocyte retrieval in IVF patients.
Methods: A retrospective observational study was conducted using medical records of IVF patients who underwent ICSI at Tiara IVF Clinic, Graha Amerta, Dr. Soetomo General Academic Hospital, Surabaya, Indonesia, between 2018 and 2025. Seventy-two eligible IVF cycles were included, consisting of 48 Day 1 ICSI cycles and 24 Day 2 ICSI cycles. Fertilization rate was classified according to laboratory performance indicators, while embryo quality was evaluated using cleavage rate and blastocyst rate. Pregnancy outcome was determined by serum β-human chorionic gonadotropin (β-hCG). Comparisons between groups were performed using the Chi-square test or Fisher's exact test, with statistical significance defined as p < 0.05.
Results: No significant difference was observed in fertilization rate between Day 1 and Day 2 ICSI groups (p = 0.474). Cleavage-stage embryo quality was significantly better in the Day 1 ICSI group than in the Day 2 group (p = 0.001). Blastocyst rate could not be statistically compared because no blastocyst data were available for the Day 2 group; therefore, this outcome was reported descriptively. Among Day 1 ICSI cycles, 32 of 48 embryos (66.7%) developed into good-quality blastocysts. Pregnancy rate was significantly higher in the Day 1 ICSI group than in the Day 2 group (p = 0.001).-
Conclusion: Day 1 ICSI was associated with superior cleavage-stage embryo quality and higher pregnancy rates compared with Day 2 ICSI, whereas fertilization rate was comparable between groups. The effect of ICSI timing on blastocyst development could not be determined because of the absence of blastocyst data in the Day 2 group. Larger prospective studies are warranted to confirm these findings.