Embryo re-expansion does not affect clinical pregnancy rates in frozen embryo transfer cycles: a retrospective study

Developing and validating a prediction model of live birth following single vitrified-warmed blastocyst transfer

RESEARCH QUESTION

Can the developed clinical prediction model offer an accurate estimate of the likelihood of live birth, involving blastocyst morphology and vitrification day after single vitrified-warmed blastocyst transfer (SVBT), and therefore assist clinicians and patients?

STUDY DESIGN

Retrospective cohort study conducted at a Spanish university-based reproductive medicine unit (2017-2021) including consecutive vitrified-warmed blastocysts from IVF cycles. A multivariable logistic regression incorporated key live birth predictors: vitrification day, embryo score, embryo ploidy status and clinically relevant variables, i.e. maternal age.

RESULTS

The training set involved 1653 SVBT cycles carried out between 2017 and 2020; 592 SVBT cycles from 2021 constituted the external validation dataset. The model revealed that female age and embryo characteristics, including overall quality and blastulation day, is linked to live birth rate in SVBT cycles. Stratification by vitrification day and quality (from day-5A to day-6 C blastocysts) applied to genetically tested and untested embryos. The model's area under the curve was 0.66 (95% CI 0.64 to 0.69) during development and 0.65 (95% CI 0.61 to 0.70) in validation, denoting moderate discrimination. Calibration plots showed strong agreement between predicted and observed probabilities.

CONCLUSION

By incorporating essential predictors such as vitrification day, embryo morphology grade, age and preimplantation genetic testing for aneuploidy usage, this predictive model offers valuable guidance to clinicians and patients, enabling accurate forecasts of live birth rates for any given vitrified blastocyst within SVBT cycles. Additionally, it serves as a potentially indispensable laboratory tool, aiding in selecting the most promising blastocysts for optimal outcomes.

Preimplantation genetic testing for aneuploidy: predictive embryonic factors

PURPOSE

In a preimplantation genetic testing for aneuploidy (PGT-A) cycle, does the blastocyst quality before biopsy, or the day of biopsy, or the embryo hatching status have an impact on either euploidy or the rate of embryo survival after freezing?

METHODS

This was a retrospective study including 6130 biopsied blastocysts coming from 1849 PGT-A cycles performed in our center (2016-2022). Embryos were categorized according to the inner cell mass and trophectoderm quality, using Gardner's scoring (excellent: AA; good: AB, BA, BB; poor: AC, CA, BC, CB, CC); the day of biopsy (5 or 6); and their hatching status (fully hatched blastocysts [FHB] or non-fully hatched blastocysts [nFHB]). The independent relationship between each group and both euploidy and survival rate was assessed.

RESULTS

Excellent-quality embryos were more euploid than both good- and poor-quality embryos (52.69%, 39.69%, and 26.21%; p < 0.001), and day 5-biopsied embryos were more euploid than day 6-biopsied embryos (39.98% and 34.80%; p < 0.001). Survival rates of excellent-quality (92.26%) and good-quality (92.47%) embryos were higher than survival rates in the poor-quality group (84.61%) (p = 0.011 and p = 0.002). Day 5-biopsied embryos survived better than day 6-biopsied embryos (93.71% vs. 83.69%; p < 0.001) and FHB had poorer survival than nFHB (78.61% vs. 93.52%; p < 0.001).

CONCLUSIONS

Excellent-quality and day 5-biopsied embryos are more prone to be euploid than good and poor or day 6-biopsied embryos, respectively. Poor-quality, day 6-biopsied embryos, and FHB have significantly lower survival after biopsy and vitrification.

Using blastocyst re-expansion rate for deciding when to warm a new blastocyst for single vitrified-warmed blastocyst transfer

RESEARCH QUESTION

Can predictive post-warm parameters that support the decision to transfer a warmed blastocyst or to warm another blastocyst be identified in women with multiple frozen-vitrified blastocysts?

DESIGN

Retrospective single-centre observational cohort analysis. A total of 1092 single vitrified-warmed blastocyst transfers (SVBT) with known Gardner score, maternal age and live birth were used to develop live birth prediction models based on logistic regression, including post-warm re-expansion parameters. Time-lapse incubation was used for pre-vitrification and post-warm embryo culture. A dataset of 558 SVBT with the same inclusion criteria was used to validate the model, but with known clinical pregnancy outcome instead of live birth outcome.

RESULTS

Three different logistic regression models were developed for predicting live birth based on post-warm blastocyst re-expansion. Different post-warm assessment times indicated that a 2-h post-warm culture period was optimal for live birth prediction (model 1). Adjusting for pre-vitrification Gardner score (model 2) and in combination with maternal age (model 3) further increased predictability (area under the curve [AUC] = 0.623, 0.633, 0.666, respectively). Model validation gave an AUC of 0.617, 0.609 and 0.624, respectively. The false negative rate and true negative rate for model 3 were 2.0 and 10.1 in the development dataset and 3.5 and 8.0 in the validation dataset.

CONCLUSIONS

Clinical application of a simple model based on 2 h of post-warm re-expansion data, pre-vitrification Gardner score and maternal age can support a standardized approach for deciding if warming another blastocyst may increase the likelihood of live birth in SVBT.

Pre-Vitrification and Post-Warming Variables of Vitrified-Warmed Blastocysts That Are Predictable for Implantation

Human IVF embryos that are not used for fresh transfer are cryopreserved by vitrification for later embryo transfers. This study evaluates pre-vitrification and post-warming embryo characteristics that are suitable to predict the chance of clinical pregnancy in single vitrified blastocyst transfer (SVBT) cycles. In a multicenter observational trial (IMBOS trial), embryos were cultured in a time-lapse system before and after vitrification. Associations between clinical pregnancy, morphokinetic parameters, blastocyst collapse, KIDScore D5, pre-vitrification and post-warming Gardner scores, post-warming blastocyst size and re-expansion rates before SVBT were analyzed in 182 SVBTs which resulted in 89 clinical pregnancies. No association was found between clinical pregnancy after SVBT and the number of collapses or the maximal collapse size before vitrification. The multifactorial analysis of pre-vitrification Gardner scores showed a significant association with clinical pregnancy for trophectoderm grading but not for expansion/hatching status and inner cell mass grading. A significant association with clinical pregnancy was found for the time to reach a blastocyst after pronuclear fading (tB-tPNf), KIDScore D5 and post-warming size but not the rate of expansion or maximal expansion size. The selection of blastocysts for SVBT could benefit from using pre-vitrification parameters like tB-tPNf, trophectoderm grading and post-warming blastocyst size.

Post-Thaw Day 5 Blastocyst Culture Time Prior to Transfer Does Not Affect Assisted Reproduction Technology (ART) Outcomes in Frozen-Thawed Embryo Transfer Cycles

The frozen embryo transfer (FET) technique has been progressively used more worldwide due to improved culture conditions, as well as enhanced survival rates after vitrification. However, little is known about the effect of the post-thaw blastocyst culture duration prior to transfer on live birth rate in FET cycles. In this retrospective observational study, we evaluated the influence of two distinct post-thaw blastocyst culture spans (2-4 h versus 20-22 h) on clinical pregnancy and live birth rate. A total of n = 1927 frozen-warmed cycles were included in the analysis. Among those, n = 885 warmed blastocysts were cultured for 2-4 h, and n = 1029 were kept in culture for 20-22 h prior to transfer; the remaining blastocysts did not survive the warming protocol. We observed no significant differences in live birth and clinical pregnancy rates between the two groups. The blastocyst morphological evaluation at transfer improved following the longer culture time. No differences between the two groups were found also for gestational and neonatal outcomes. This work shows that different post-thaw embryo culture timings do not negatively impact pregnancy outcomes. Overall, these results are important in the context of the embryological laboratory in order to better organize the workflow and avoid unnecessary timing-related workload.

Clinical Usability of Embryo Development Using a Combined Qualitative and Quantitative Approach in a Single Vitrified-Warmed Blastocyst Transfer: Assessment of Pre-Vitrified Blastocyst Diameter and Post-Warmed Blastocyst Re-Expansion Speed

Improving the safety and efficacy of assisted reproductive technology programs has been a continuous challenge. Traditionally, morphological grading has been used for embryo selection. However, only a few studies have assessed the morphokinetic variables and morphological dynamics of blastocysts. In the present study, we aimed to perform a quantitative analysis of blastocyst diameter and re-expansion speed. This in-depth morphokinetic evaluation can correlate with currently observed pregnancy outcomes. In total, 658 single vitrified-warmed blastocyst transfer cycles were performed between October 2017 and December 2021, which were divided into four groups according to the pre-vitrified blastocyst diameter. After warming, the groups were subdivided according to the blastocyst re-expansion speed. These quantitative measurements were performed using a time-lapse system. Both diameter and speed are essential in determining the blastocyst quality, while age, day of freezing, and blastocyst quality are crucial from a clinical perspective. The application of both quantitative (diameter and speed) and qualitative (blastocyst quality scores) parameters can help evaluate the clinical usability of blastocysts. This method can prove useful for embryologists in counseling their patients and determining pregnancy patient-oriented strategies.

Standardization of Post-Vitrification Human Blastocyst Expansion as a Tool for Implantation Prediction

The increased use of vitrified blastocysts has encouraged the development of various criteria for selecting the embryo most likely to implant. Post-thaw assessment methods and timetables vary among investigators. We investigated the predictive value of well-defined measurements of human blastocyst re-expansion, following a fixed incubation period. Post-thaw measurements were taken exactly at 0 and 120 ± 15 min. Minimum and maximum cross-sectional axes were measured. Three groups were defined: Group 1: embryos that continued to shrink by 10 µm or more; group 2: embryos that ranged from −9 to +9 µm; and group 3: re-expansion of 10 µm or more. Patient and morphokinetic data were collected and integrated into the analysis. A total of 115 cases were included. The clinical pregnancy rate for group 1 was 18.9%; group 2, 27%; and group 3, 51.2% (p = 0.007). Pre-thaw morphologic grading and morphokinetic scores of the study groups did not reveal differences. p-values were 0.17 for the pre-thaw morphologic score, 0.54 for KID3, and 0.37 for KID5. The patients’ demographic and clinical data were similar. The clinical pregnancy rate correlated with the degree of thawed blastocyst re-expansion measured 2 h after incubation. This standardized measure is suggested as a tool to predict the potential of treatment success before embryo transfer.

Post-warming embryo morphology is associated with live birth: a cohort study of single vitrified-warmed blastocyst transfer cycles

Purpose

This study aims to examine whether blastocyst morphology post-warming correlates with live birth.

Methods

In this cohort study, morphological characteristics post-warming were reviewed in all single vitrified-warmed blastocyst transfer cycles performed between November 2016 and May 2017. Immediately before transfer, the degree of blastocoel re-expansion was graded as A, fully expanded; B, partially expanded ≥ 50%; C, partially expanded < 50%; and D, collapsed. The degree of post-warming cell survival was graded on a scale of 50 to 100% and was then classified into 4 groups: very low 50–70%, low 71–80%, moderate 81–90%, and high 91–100%.

Results

Overall, 612 cycles were reviewed, of which 196 included PGT-A tested embryos. The live birth rate (LBR) increased from 11.4% in the collapsed blastocysts group to 38.9% in the post-warming full re-expansion group (p < 0.001) and from 6.5% for blastocysts with a very low cell survival rate to 34.7% for blastocysts with high cell survival rate (p = 0.001). LBR was 6.7% for blastocysts with the worst post-warming morphological characteristics, namely, collapsed with very low cell survival rate. On multivariate analyses, partial blastocyst re-expansion ≥ 50%, full re-expansion, and high cell survival rate remained significantly associated with live birth, after controlling for female age, pre-vitrification morphological grading, and PGT-A. A sub-analysis of cycles using PGT-A tested embryos showed similar results.

Conclusion

Post-warming re-expansion and high cell survival rate are associated with higher LBR in euploid and untested blastocysts. However, embryos with poor post-warming morphology still demonstrate a considerable probability of live birth, and they should not be discarded.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10815-021-02390-z.