Does Blastocyst-To-Oocyte Ratio Predict Live Birth When Only One Blastocyst Is Available For Transfer?

Objective: This study aimed to evaluate whether having only one blastocyst-stage embryo on Day 5/6 rectify the live birth rate (LBR) when various number of oocytes had been collected.

Design: A retrospective cohort study from two in vitro fertilization (IVF) centers has been conducted. Participants/Materials: The study included women undergoing IVF treatment whose cycles resulted in only one blastocyst-stage embryo available for frozen transfer on Day 5/6. Cases with no oocyte retrieval, no blastocyst development, or missing clinical data were excluded. There were no restrictions based on female age or BMI to reflect real-world clinical conditions.

Setting: A multi-center study was conducted.

Methods: This retrospective cohort study included 2,125 single blastocyst frozen embryo transfer cycles performed between November 2018 and February 2023. All patients had only one blastocyst-stage embryo available for transfer on Day 5/6, regardless of the number of oocytes retrieved during controlled ovarian stimulation. Patients were stratified into quartiles based on their blastocyst-to-oocyte ratio. Baseline demographic, ovarian stimulation, and embryological parameters were compared across quartiles. The primary outcome was the LBR. Binary logistic regression was used to identify independent predictors of the LBR, including female age, embryo quality, BMI, and blastocyst-to-oocyte ratio.

Results: The mean blastocyst-to-oocyte ratio was 18.6%. Patients in the lowest quartile had significantly younger mean age and higher AMH levels compared to the highest quartile. Although blastocyst development rates increased across quartiles, the LBR was lower in the highest quartile from all other groups (24.5% vs. 31.9 to 29.9%). When the LBR was analyzed as dependent variable, binary logistic regression identified female age (β = 0.93, 95% CI: 0.92-0.95, p < 0.001) and embryo quality (β = 2.35, 95% CI: 1.62-3.39, p < 0.001, compared with moderate-quality embryos; β = 4.22, 95% CI: 2.91-6.11, p < 0.001, compared with poor-quality embryos) as independent predictors. However, the blastocyst-to-oocyte ratio did not demonstrate a significant association with the LBR.

Limitations: The retrospective design and absence of genetic testing for embryo ploidy might limit the ability to establish causality. Variability in laboratory conditions and stimulation protocols may also have introduced confounding factors.

Conclusions: The blastocyst-to-oocyte ratio does not significantly impact the LBR when only one blastocyst is available for transfer. Instead, female age and embryo quality remain the most critical factors in determining the LBR. These findings emphasize the importance of embryo selection over numerical ovarian response parameters in clinical decision-making to obtain live birth.