Blastocyst

Last updated: March 2026

What is a blastocyst?

A blastocyst is “a ball of cells that forms early in a pregnancy, about five to six days after a sperm fertilises an egg,” containing approximately 200-300 cells (Cleveland Clinic, 2024). It consists of two layers: the inner cell mass, which develops into the fetus, and an outer layer that forms the placenta (Cleveland Clinic, 2024).

How many make it?

Not all fertilised eggs reach blastocyst stage. The ESHRE/Alpha Istanbul Consensus (2011) set a minimum competency benchmark of 40% blastocyst formation from normally fertilised (2PN) embryos. The aspirational target: 60% or above. In practice, younger patients (under 35) may see 50-70% reach blastocyst. Older patients (over 40) may see 20-40%. Your clinic’s blastocyst rate is a direct indicator of lab quality.

Why does blastocyst culture matter in IVF?

“Five or six days after fertilisation, blastocysts are ideal to transfer to your uterus during IVF” (Cleveland Clinic, 2024). Culturing to day 5 allows the laboratory to identify the strongest embryos before transfer.

A Cochrane Review (Glujovsky et al., 2022) found blastocyst transfer was associated with a higher live birth rate per cycle started compared to day-3 (cleavage-stage) transfer (OR ~1.27). The catch: when you include frozen transfers from the same cycle, the gap narrows. Extended culture selects better embryos for fresh transfer, but some viable day-3 embryos arrest before day 5 in the lab. For patients with few embryos, that risk matters.

Extended culture helped “maintain birth rates from single embryo transfer while decreasing multiple birth rates” (HFEA, 2021). This is a key reason most European clinics now favour day-5 transfers.

Grading

Most clinics use the Gardner grading system (Gardner & Schoolcraft, 1999). Three components:

• Expansion (1-6): from early cavity to fully hatched
• Inner cell mass (A-C): the cells that become the baby. A = many tightly packed cells. C = very few cells.
• Trophectoderm (A-C): the cells that become the placenta. Same scale.

A “4AA” is an expanded blastocyst with excellent ICM and trophectoderm: implantation rates of 50-65% per transfer. A “3BB” is still good (35-50%). Grading is subjective, varies between embryologists, and is one reason PGT-A exists.

Implantation

Failure of implantation accounts for approximately 75% of early miscarriages (Cleveland Clinic, 2024). Some clinics use PGT-A to screen blastocysts for chromosomal abnormalities before transfer, though its benefit is debated. Blastocysts can also be vitrified (rapidly frozen) for later frozen embryo transfer, with thaw survival rates above 95%.

What to ask your clinic

• What percentage of embryos reach blastocyst stage at your lab?
• Do you recommend day-3 or day-5 transfer for my situation?
• How do you grade blastocyst quality, and what grades are suitable for transfer or freezing?
• Do you offer extended culture to day 6-7 for patients with few embryos?

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Sources

1. Cleveland Clinic. Blastocyst: https://my.clevelandclinic.org/health/body/22889-blastocyst
2. HFEA. Fertility Treatment 2021: Preliminary Trends and Figures: https://www.hfea.gov.uk/about-us/publications/research-and-data/fertility-treatment-2021-preliminary-trends-and-figures/
3. Alpha/ESHRE. The Istanbul Consensus Workshop on Embryo Assessment. Human Reproduction, 2011;26(6):1270-1283.
4. Glujovsky et al. Cleavage-stage versus blastocyst-stage embryo transfer. Cochrane Database of Systematic Reviews, 2022.
5. Gardner DK, Schoolcraft WB. In Vitro Culture of Human Blastocysts. 1999.

This page is for informational purposes only and does not constitute medical advice. Always consult a qualified fertility specialist before making treatment decisions.