What is a blastocyst, and why day 5 matters
A blastocyst is an embryo that has developed to around day 5, forming two distinct cell types: the inner cell mass, which becomes the baby, and the trophectoderm, which becomes the placenta.
The benchmarks
From the Vienna Consensus on ART laboratory performance indicators, blastocyst development rate on day 5 — assessed at 116 ± 2 hours, per normally fertilised egg:
- Competency (minimum acceptable): at least 40%
- Benchmark (aspirational): at least 60%
A further 10 to 15% may develop by day 6. Good-quality blastocyst rate on day 5: competency at least 30%, benchmark at least 40%.
Two ways these numbers get misapplied
The denominator. The rate is per normally fertilised (2PN) egg, not per cleaved embryo. Using cleaved embryos inflates the figure and makes it non-comparable.
The timing. 116 ± 2 hours, day 5 rather than day 5 and 6 combined. Embryos assessed later appear to perform better.
If you are comparing clinics on blastocyst rates, both need to match or the comparison is meaningless.
Grading
Three components: blastocoele expansion, inner cell mass, and trophectoderm.
Of these, trophectoderm appearance was the statistically significant independent predictor of live birth after adjusting for confounders — counter-intuitive, since it becomes the placenta rather than the baby.
An important caveat
The Vienna Consensus describes embryo grading systems as not robust, and better suited to internal quality assessment than to comparison between clinics. Fragmentation assessment in particular is subjective — surveyed competence values ranged from 20% to 90%.
So a grade from one laboratory is not directly comparable to another's.
Why not all embryos reach blastocyst
Most embryos that arrest do so because of chromosomal abnormalities. Extended culture reveals this rather than causing it — an embryo that stops on day 4 would very likely not have implanted had it been transferred on day 3.
Worth knowing if you have had embryos fail to reach blastocyst. It is information, not loss caused by the laboratory.
Laboratory factors
The Vienna Consensus names timing of observation, culture medium, and culture conditions — with oxygen concentration singled out. Oxygen tension is a facility-level parameter, so a problem affects every patient at once rather than showing as individual poor outcomes.
What to ask
Your clinic's blastocyst development rate, with the denominator and assessment timing stated. And what proportion of their cycles reach transfer.
Sources
- Vienna Consensus — ART laboratory performance indicators (2017)
Frequently Asked Questions
Is a blastocyst better than a Day 3 embryo for IVF?▾
Yes — blastocysts have significantly higher implantation rates than Day 3 embryos (40–60% vs 20–35% per transfer per ESHRE 2023). Only developmentally competent embryos survive to Day 5, providing natural selection. Most IVF clinics now culture routinely to Day 5. Day 3 transfer may be preferred when few eggs were fertilised.
What is a good blastocyst grade?▾
Using the Gardner system: 4AA, 4AB, or 4BA are excellent — approximately 60–70% implantation rate. 3BB or 4BB are good — 40–50% implantation. Even 2BC or 3BC blastocysts have meaningful implantation rates (20–30%) and are worth transferring. Embryo grade predicts probability, not certainty.
What does it mean if no embryos reached blastocyst?▾
Blastocyst arrest affects about 15–20% of IVF cycles. The most common cause in women over 37 is chromosomal quality issues in eggs. It may also indicate laboratory conditions need review. Discuss with your team whether Day 3 transfer, different stimulation, or a laboratory change should be considered for future cycles.