Does blastocyst-stage (day 5-6) embryo transfer improve pregnancy and live birth rates per frozen transfer compared to cleavage-stage (day 2-3) embryo transfer?

Pregnancy rate is significantly higher following frozen blastocyst-stage embryo transfer, while live birth rate does not differ.

The extended culture of cleavage-stage (day 2-3) embryos to blastocyst-stage (day 5-6) embryos has been largely adopted in recent years. In fact, blastocyst culture may allow a better selection of the embryo to be transferred based on its developmental history and morphological criteria, and result in a lower number of exceeded embryos to be frozen for storage. However, the literature comparing pregnancy and live birth rates for cleavage- and blastocyst-stage transfer remains contradictory, and the benefits of frozen blastocyst transfer is still under discussion.

This retrospective study investigated couples having frozen embryo transfer at the cleavage (days 2 or 3) or the blastocyst-stage (days 5 or 6) at our clinic between 2014–2021. Results from more cycles were analyzed individually. We excluded cycles based on fresh embryo transfer, oocyte or sperm donation. Cycles were further sub-classified based on the type of treatment, as natural or stimulated, and the pregnancy and live birth rates were compared.

Data was obtained from patient records at our fertility clinic and sorted by two authors. Variables included female age, treatment plan, the stage of the embryo transferred, and the report of pregnancy and live birth. Statistical analysis was conducted by using MedCalc Software, and Chi-squared test applied to compare the outcomes’ frequency based on the day of frozen embryo transfer. P < 0.05 was considered significant.

A total of 2,922 cycles were included for analysis, with age of female patients being 37.2 ± 5.0 years at the time of transfer. Most cycles were stimulated (n = 2,217, 75.9%) compared to natural cycles (n = 705, 24.1%). In all cycles, IVF was used for fertilization. Approximately one third of the cycles (n = 954, 32.6%) underwent frozen embryo transfer at the cleavage-stage (days 2 or 3), while the remaining had transfer at the blastocyst-stage (n = 1,968, 67.4%).

Pregnancy rate was significantly (P < 0.0001) higher for blastocyst-stage embryo transfer compared to cleavage-stage embryo transfer (36.2% vs 20.4%, respectively) when all cycles were analyzed. This significance was found when natural (23.3% vs 14.8%, P = 0.0052) or stimulated cycles (38.0% vs 25.8%, P < 0.0001) were sub-analyzed.

While we observed 19.0% of successful live birth per embryo transferred in the entire cohort, this did not significantly vary depending on the stage of frozen embryo transferred (cleavage or blastocyst-stage, 13.6% vs 21.5%, respectively) and when patients were sub-classified based on the type of treatment chosen in natural (9.0% vs 14.2% respectively) or stimulated (18.0% vs 22.6%, respectively) cycles.

With a large sample size and a very significant P-value for pregnancy rate, the role of chance is limited.

As a retrospective study, the non-random allocation to treatment groups might hide selection bias and limit the statistical power of the data analysis, especially for the live birth rate. Also, no adjustment for confounders was performed.