In the present study, we developed a model for predicting the individual probability of usable blastocyst formation on Day 5 based on training cohorts and then the model was validated by validation cohort. The model indicated that female age, type of fertilization, fertilization rate, cleavage rate, number of Day 3 embryos extended culture to blastocyst stage, and high-quality rate of Day 3 embryos extended culture to blastocyst stage were predictors of usable blastocysts formation on Day 5.
Two previously study have developed models to predict blastocyst formation on Day 5(Dessolle et al., 2009; Jin et al., 2021), however our prediction model has some advantages compared to the two studies. First, the discrimination of our predict model was better than these previously predicted models. In the present the AUC value of training cohort and validation cohorts were 0.874 (95% CI: 0.862–0.886) and 0.886 (95% CI: 0.867–0.905) respectively, showing good discrimination ability of the model; while the AUC values of training cohorts of these previously predicted models were fair performance. Second, the accuracy of our model was better than the model developed by Dessolle L et al., as the calibration curves of training cohort and validation were close to the ideal diagonal line in the present study while the calibration curve of validation cohort was poor performance (P < 0.01) in Dessolle L et al. 's study. Third, our prediction model could be used for predicting blastocyst formation in both IVF and ICSI cycles while models of Jin H et al. 's study are only suitable for IVF cycles.
Previous studies have shown that female age is strongly associated with the clinical outcomes of reproductive medicine (van Loendersloot et al., 2010; von Wolff et al., 2019). Some studies have shown that increased female age has a negative effect on blastocyst formation (Dessolle et al., 2009; Jin et al., 2021; La Marca et al., 2022; Thomas et al., 2010). In our nomogram model, a lower female age corresponding to higher points indicated a higher probability of blastocyst formation on Day 5; these results were consistent with the published data. It was well known, abnormalities in the oocyte ooplasm, including organelle dysfunction, altered metabolism, and aberrant gene regulation, would be accumulative with increased female age and progressively undermined oocyte quality (Bebbere et al., 2022). Oocytes derived from older woman would reduce the ability to repair sperm DNA fragmentation which was negatively correlated with blastocyst formation(Sedó et al., 2017; Setti et al., 2021). Moreover, mitochondrial function at morula stage was decreased with maternal ageing, and thus impar morula-to-blastocyst transition(Hashimoto and Morimoto, 2022). A recent study has indicated that advanced maternal age significantly effect on pronuclear, chromatin dynamics, regulation of cell polarity and blastocyst formation rate (Ezoe et al., 2023).
Type of fertilization was also an independent predictor in our model. In the present study, multivariate regression analysis showed that type of fertilization was associated with the formation of usable blastocysts on Day 5 (ICSI vs. IVF) (P < 0.01, OR 0.612, 95% CI: 0.474–0.789). This result was in accordance with some earlier published studies (Dessolle et al., 2009; Thomas et al., 2010; Yin et al., 2014). Thomas et al. indicated that ICSI has a negative effect on blastocyst formation rate (Thomas et al., 2010). Yin et al. found that either the blastocyst formation rate where blastocyst derived from good morphology embryos or the total blastocyst formation rate in IVF cycles was significantly higher compared to ICSI cycles (Yin et al., 2014). This phenomenon could be explained that ICSI technology can bypass the selective biological barrier of zona pellucida and may bring abnormal sperm into the oocyte, subsequently affecting embryonic development and blastocyst formation. One study has determined that the ICSI zygotes have many more vacuoles than IVF zygotes, and the presence of vacuoles was related to a lower blastocyst formation rate (Ebner et al., 2005).
In the present study, number of Day 3 embryos extended culture to blastocyst stage was significant associated with usable blastocyst formation on Day 5 and was included in the model. Surprisingly, the number of oocytes retrieved was not incorporated into the model. A systematic review and meta-analysis has indicated that the number of oocytes retrieved was positively correlated with the number of high-quality embryo (Vermey et al., 2019). Moreover, two studies that construct prediction models for predicting blastocyst formation have demonstrated that the number of oocytes retrieved was an important predictor of blastocyst formation (Dessolle et al., 2009; Jin et al., 2021). The causation of the number of Day 3 embryos extended culture to blastocyst stage rather than the number of oocytes retrieved entered in the model, may be attributed to the fact that not all Day 3 embryos cohort were cultured to blastocyst stage and usually the best one or two embryos were transferred or vitrified in the present study.
The fertilization rate, cleavage rate, high-quality rate of Day 3 embryos extended culture to blastocyst stage were residual predictors of usable blastocyst formation on Day 5. Higher fertilization rate or cleavage rate usually produce more zygotes and cleaved embryos in fresh cycles, earlier study has demonstrated that the number of zygotes and cleaved embryos have positive effect on clinical outcomes(Hariton et al., 2017). It was shown that high-quality Day 3 embryos have more potential to develop into blastocysts(Yin et al., 2014), in the present study high-quality rate of Day 3 embryos extended culture to blastocyst stage corresponded to a high point; these results were similar to the previously predicted models(Dessolle et al., 2009; Jin et al., 2021).
In the present study, we developed a prediction model with good discrimination and accuracy. Nevertheless, the study has two limitations. First, the present study was a retrospective study, and hence the probability of potential bias could not be excluded. Second, the data analyzed in the present study was from a single center; thus, a multicenter study should be conducted to test the applicability of the model future.
In conclusion, we developed a model to predict the probability of usable blastocysts formation on Day 5. Variables, including female age, type of fertilization, fertilization rate, cleavage rate, number of Day 3 embryos extended culture to blastocyst stage, and high-quality rate of Day 3 embryos extended culture to blastocyst stage were entered into this model. This prediction model performed satisfactorily, as confirmed by validation data. Thus, it provides an intuitive and simple tool for predicting the probability of usable blastocyst formation on Day 5, which might be helpful in decreasing the cancellation rate of blastocyst transfer.