Most embryos that develop into blastocysts do so 5 days after fertilisation. Traditionally, this is the stage that has been considered the most likely to result in a successful pregnancy. However, some embryos take a day longer to reach the same stage, but may still be suitable for transfer.
A new study* aimed to investigate whether outcomes were comparable between day 5 and day 6 blastocysts.
Researchers from the University Hospital Zurich analysed 631 frozen single blastocyst transfers carried out between 2017 – 2023. 541 of the blastocysts were frozen on day 5 and 90 on day 6. The blastocysts were all derived from patients within a similar age group and PGT-a was not done to compare the quality of embryos.
Initially, results appeared to be better for day 5 embryos, with a live birth rate of 30.3% by comparison to 20.0% for day 6. However, when adjustments were made for embryo quality and other factors that could have influenced success, day 6 embryos were found to be no longer significantly associated with a lower chance of a live birth. There were also no significant differences in clinical pregnancy, biochemical pregnancy or miscarriage rates.
These findings suggest that blastocysts should not necessarily be considered to have less potential for success simply because they took an additional day to develop. Other factors may be more relevant than whether the embryo became a blastocyst on day 5 or day 6, and day 6 blastocysts still have the potential to result in successful pregnancies and healthy live births.
It could also mean that for patients who do not have any blastocysts successfully develop by day 5, they will still have the possibility of viable embryos available to transfer by waiting for an additional day of development.
However, it should be noted that this study was a single-centre retrospective study, and the number of day 6 blastocysts included in it was relatively low. Larger studies would help to confirm the findings conclusively.
NOW-fertility’s team strongly believes that individualised decision-making in embryo transfer involves assessing each embryo according to quality, developmental stage, and each patient’s clinical circumstances. Embryo morphology and development potential should be always considered when selecting the most appropriate embryo for transfer. If PGT-a is used after embryo biopsy on day 5 then a fresh embryo transfer on day 6 may take in to account the genetic status of the embryo too.
NOW-fertility’s approach is to maximise the chances of a successful outcome with healthy pregnancy in the shortest possible time.
Â
*Carullo G, et al. (2026). Comparable live births for single vitrified blastocyst transfer from day 5 compared with day 6: a retrospective analysis. Reproductive BioMedicine Online, 53(4), 105796.
