Blastocyst Culture and Grading Explained: A Practical Guide for Trainee Embryologists

Extended culture to the blastocyst stage is now routine in many IVF laboratories. For trainee embryologists, learning to assess and grade blastocysts consistently is a core skill - and one that improves only with structured practice and regular comparison with experienced colleagues.

Embryo development at a glance

After fertilisation is confirmed (usually around 16-18 hours after insemination, by the presence of two pronuclei), the embryo develops through recognisable stages:

• Day 2-3: cleavage stage - the embryo divides into roughly 2-4 cells on day 2 and 6-8 cells on day 3
• Day 4: morula - the cells compact together
• Day 5-6: blastocyst - a fluid-filled cavity (blastocoel) forms, and the cells separate into the inner cell mass (which forms the fetus) and the trophectoderm (which forms the placenta)

Why culture to blastocyst?

Culturing to day 5 or 6 lets the embryologist observe which embryos keep developing, which can help with embryo selection and supports single embryo transfer. It also provides the stage at which trophectoderm biopsy is performed when preimplantation genetic testing is used. Whether extended culture suits a particular patient is a clinical and laboratory decision made case by case.

The Gardner grading system

The most widely used system, described by Gardner and Schoolcraft, combines a number and two letters.

Expansion stage (1-6)

1. Early blastocyst - the cavity is less than half the volume of the embryo
2. Blastocyst - the cavity is half or more of the embryo
3. Full blastocyst - the cavity fills the embryo
4. Expanded blastocyst - the cavity is larger than the original embryo and the zona is thinning
5. Hatching blastocyst - trophectoderm is herniating through the zona
6. Hatched blastocyst - the blastocyst has fully escaped the zona

Inner cell mass (A-C)

• A - many cells, tightly packed
• B - several cells, loosely grouped
• C - very few cells

Trophectoderm (A-C)

• A - many cells forming a cohesive layer
• B - few cells forming a loose layer
• C - very few, large cells

A blastocyst graded 4AA, for example, is expanded with a tightly packed inner cell mass and a cohesive trophectoderm. ICM and trophectoderm grades are usually assigned only from stage 3 onwards.

Practical tips for consistent grading

• Time your assessments - grade at consistent times after insemination, as blastocysts change quickly
• Minimise time out of the incubator - work efficiently to protect temperature and pH
• Use multiple focal planes - focus through the embryo to see the ICM and trophectoderm properly
• Grade independently, then compare - regular inter-observer comparison within the lab is one of the best ways to reduce variation
• Document carefully - record grades, timings and images where your system allows

Time-lapse imaging

Time-lapse incubators photograph embryos at regular intervals without removing them from culture, allowing morphokinetic assessment. They are a useful tool, but trainees still need a solid foundation in conventional morphological grading.

Common mistakes trainees make

• Assigning ICM and TE grades too early, before the cavity is properly formed
• Mistaking a collapsed blastocyst for a poor-quality embryo - blastocysts can collapse and re-expand
• Inconsistent terminology between staff, which makes records hard to compare

Learn grading at the bench

Grading is learned by doing - looking at many embryos under supervision and discussing each assessment. ReproNest's Clinical Embryology & IVF Lab Training includes structured practice in embryo assessment, alongside gamete handling and vitrification. Read our guide on how to become a clinical embryologist in India, or contact us about the next batch.