Design:
Retrospective observational study.
Pinto E. Effects of Delayed Blastulation on Clinical Pregnancy and Implantation Rates with Single Frozen Blastocyst Transfer. Poster presented at the ASPIRE (Asia Pacific Initiative on Reproduction) 15th Congress, 7-10 May 2026, China National Convention Center, Beijing, China. KL Fertility & Gynaecology Centre, Kuala Lumpur, Malaysia.
Presented: ASPIRE 2026, Beijing | Last reviewed: August 2026
This page summarises findings from a conference poster presentation based on clinical data from KL Fertility & Gynaecology Centre. It is presented here for the clinical and scientific community and is not intended as a substitute for individualised patient counselling.
Funding and disclosures: No funding declarations are available for this conference poster. The author reports no competing interests.
Background
Blastocyst formation is expected to occur on Day 5 of embryo development under standard in vitro culture conditions. Delayed blastulation, defined as blastocyst formation on Day 6 or Day 7, is commonly observed in clinical practice. Whether this developmental delay is associated with reduced reproductive competence, or whether Day 6 and Day 7 blastocysts remain viable candidates for frozen embryo transfer, has been the subject of ongoing debate in the reproductive medicine literature.
The potential mechanisms underlying reduced outcomes with delayed blastulation include impaired genomic reprogramming, higher rates of chromosomal aneuploidy, reduced mitochondrial function, and lower intrinsic developmental potential. This study aimed to investigate the influence of delayed blastulation on clinical pregnancy rate (CP) and implantation rate (IR) in single frozen blastocyst transfer (SBT) cycles at KL Fertility & Gynaecology Centre.
Method: Study Design and Patient Population
Subjects:
Patients aged 35 to 45 who underwent single blastocyst transfer (SBT) from January 2023 to May 2025 at KL Fertility & Gynaecology Centre. Cycles were performed under both natural cycle and hormone replacement therapy (HRT) protocols.
Exclusion criteria:
Cases involving assisted hatching, embryo glue, genetically tested embryos (PGT), and donated gametes or embryos.
Study groups:
Day 5 blastocysts (n=56):
Blastocysts reaching full blastocyst stage by Day 5 of development.
Day 6 blastocysts (n=75):
Blastocysts reaching full blastocyst stage by Day 6 of development.
Day 7 blastocysts (n=30):
Blastocysts reaching full blastocyst stage by Day 7 of development.
Total SBT cycles: n=161.
Outcome measures:
Clinical pregnancy rate (CP) and implantation rate (IR) per SBT cycle. Note: the poster does not explicitly define the endpoints for CP and IR. Given that IR exceeds CP across all three groups, the most plausible interpretation is that CP represents ongoing pregnancy at 12 weeks (the stricter endpoint) and IR represents intrauterine sac seen on ultrasound at approximately 6 weeks (the earlier endpoint). The difference between IR and CP in each group thus reflects early pregnancy loss between 6 and 12 weeks. This interpretation should be confirmed with the author before publication.
Statistical analysis:
ANOVA and Chi-square (χ²) tests. Statistical significance defined as p ≤ 0.05.
Results: Clinical Pregnancy and Implantation Rates by Blastocyst Day
Table 1. Clinical pregnancy and implantation rates by blastocyst developmental day
Group | n | Clinical pregnancy rate (CP) | Implantation rate (IR) |
|---|---|---|---|
Day 5 blastocysts | 56 | 49.2% | 64.6% |
Day 6 blastocysts | 75 | 29.5% | 40.9% |
Day 7 blastocysts | 30 | 5.3% | 10.5% |
A significant decline in both CP and IR was observed across Day 5, Day 6, and Day 7 SBT groups (p < 0.001 for the overall comparison). All pairwise comparisons reached statistical significance. The greatest difference was observed between Day 5 and Day 7 transfers:
- CP: Day 5 vs. Day 7: 49.2% vs. 5.3% (χ²=20.964; p < 0.001)
- IR: Day 5 vs. Day 7: 64.6% vs. 10.5% (χ²=28.387; p < 0.001)
Discussion: How These Findings Compare With Published Evidence
The progressive decline in clinical pregnancy and implantation rates from Day 5 to Day 6 to Day 7 blastocyst transfer is consistent with a substantial and growing body of evidence:
- A 2023 large matched-cohort study found significantly higher clinical pregnancy and live birth rates in Day 5 compared with Day 6 frozen blastocyst transfers, with this difference holding across embryo quality grades.
- A systematic review and meta-analysis of frozen-thawed Day 7 blastocyst transfer found Day 7 blastocysts were associated with significantly reduced clinical pregnancy rate compared with Day 5/6 transfers (OR 0.36 vs. Day 5; OR 0.43 vs. Day 6, both p < 0.001), a substantially reduced live birth rate (OR 0.21 vs. Day 5; OR 0.34 vs. Day 6), and a significantly lower euploid rate, suggesting that delayed blastulation frequently reflects underlying aneuploidy rather than merely slower development of a competent embryo.
- A 2025 study on single frozen euploid blastocyst transfers (PGT-A screened) found Day 5 euploid blastocysts had significantly higher implantation rates (68.8% vs. 48.3%) and live birth rates (63.7% vs. 40.4%) than Day 6 euploid blastocysts. The developmental speed disadvantage was more pronounced in older patients and those with lower-quality embryos, suggesting the impact of delayed blastulation is not uniform across patient groups.
- The present study's CP rates (49.2% for Day 5, 29.5% for Day 6, 5.3% for Day 7) align with published benchmarks for an older patient cohort (35-45 years), where age-related aneuploidy rates compound the developmental delay effect.
The conclusion that delayed blastulation could indicate reduced embryo quality or increased genetic abnormality is well supported by the literature, particularly for Day 7 blastocysts.
Clinical Significance
These findings have direct implications for embryo selection and patient counselling in clinical IVF practice:
For embryologists and clinicians:
- Day 5 blastocysts should be preferentially selected for transfer where available, as they are consistently associated with the highest clinical pregnancy and implantation rates.
- Day 6 blastocysts remain a clinically viable option, albeit with meaningfully lower expected outcomes. The literature supports cryopreservation and transfer of Day 6 blastocysts, particularly when no Day 5 blastocysts are available.
- Day 7 blastocysts show markedly reduced CP and IR in this cohort. Published evidence increasingly points to a high aneuploidy burden in Day 7 blastocysts as the primary driver; PGT-A screening may help identify the minority of chromosomally normal Day 7 embryos that retain transfer viability.
These findings have direct implications for embryo selection and patient counselling in clinical IVF practice:
For embryologists and clinicians:
- Day 5 blastocysts should be preferentially selected for transfer where available, as they are consistently associated with the highest clinical pregnancy and implantation rates.
- Day 6 blastocysts remain a clinically viable option, albeit with meaningfully lower expected outcomes. The literature supports cryopreservation and transfer of Day 6 blastocysts, particularly when no Day 5 blastocysts are available.
- Day 7 blastocysts show markedly reduced CP and IR in this cohort. Published evidence increasingly points to a high aneuploidy burden in Day 7 blastocysts as the primary driver; PGT-A screening may help identify the minority of chromosomally normal Day 7 embryos that retain transfer viability.
For embryologists and clinicians:
- Patients in the 35-45 age group whose embryos reach blastocyst stage on Day 6 or Day 7 should be counselled that outcomes are expected to be lower than for Day 5 blastocysts, with the drop being particularly pronounced for Day 7. This does not mean Day 6 or Day 7 embryos cannot result in a healthy pregnancy, but realistic expectations are important given the data.
Limitations of This Retrospective Study
- Retrospective, single-centre, non-randomised design: patient and cycle characteristics may differ systematically between the three groups in ways the study design cannot fully control for.
- The age range of 35 to 45 years is an older cohort than many comparable studies. This amplifies the aneuploidy effect and limits direct comparison with studies in younger patients.
CP and IR definitions are not explicitly stated in the poster. The interpretation of CP as ongoing pregnancy (12 weeks) and IR as gestational sac (6 weeks) is inferred from the observed pattern (IR > CP across all groups) and should be confirmed with the author.
- No data on blastocyst morphological quality (Gardner grade), embryo quality stratification, or endometrial preparation protocol breakdown are provided. These are recognised confounders of blastocyst transfer outcomes.
- Day 7 blastocysts have a small sample size (n=30), which limits the precision of the 5.3% CP and 10.5% IR estimates.
- No live birth data are reported. Clinical pregnancy rate is a proxy outcome.
- Conference poster format; findings have not yet been subject to full peer-review.
Key Findings
- In 161 single frozen blastocyst transfer cycles (ages 35-45), clinical pregnancy and implantation rates declined significantly with each day of developmental delay: 49.2%/64.6% for Day 5, 29.5%/40.9% for Day 6, and 5.3%/10.5% for Day 7 (p < 0.001 overall; all pairwise comparisons significant).
- The greatest decline was between Day 5 and Day 7 transfers: CP difference 43.9 percentage points (χ²=20.964; p < 0.001), IR difference 54.1 percentage points (χ²=28.387; p < 0.001).
- The decline likely reflects higher rates of chromosomal aneuploidy and reduced developmental competence in blastocysts reaching the blastocyst stage later in culture.
- Day 5 blastocysts should be preferentially selected for transfer where available.
- Day 6 blastocysts remain a viable option with lower but acceptable outcomes; Day 7 blastocysts show substantially reduced outcomes and may benefit from PGT-A screening.
About the Researcher
- Pinto E conducted this research as part of the Embryology team at KL Fertility & Gynaecology Centre.
References
- Pinto E. Effects of Delayed Blastulation on Clinical Pregnancy and Implantation Rates with Single Frozen Blastocyst Transfer. Poster presented at the ASPIRE 15th Congress, Beijing, China, May 2026.
- Yu Q, He H, Ren XL, et al. Pregnancy Outcomes for Day 5 Versus Day 6 Single Frozen-thawed Blastocyst Transfer with Different Qualities of Embryos: A Large Matched-cohort Study. Curr Med Sci 2023;43:297-303. https://doi.org/10.1007/s11596-023-2699-4
- Abdala A, Elkhatib I, Bayram A, et al. Clinical Outcomes Deriving from Transfer of Blastocysts Developed in Day 7: a Systematic Review and Meta-Analysis of Frozen-Thawed IVF Cycles. Reprod Sci 2021;28(4):939-947. https://doi.org/10.1007/s43032-020-00424-y
- Lang J, Hong L, Yang W, et al. Delayed blastulation and pregnancy outcomes in single frozen-thawed euploid blastocyst transfer. Front Endocrinol 2025. https://doi.org/10.3389/fendo.2025.1686274
- Zhan S, Lin C, Lin Q, et al. Obstetric, neonatal, and clinical outcomes of Day 6 vs. Day 5 vitrified-warmed blastocyst transfers: retrospective cohort study with propensity score matching. Hum Reprod 2024;39(9):1960-1968. https://doi.org/10.1093/humrep/deae150