The role of embryo grading in selection
Embryo grading is a structured, morphological appraisal used to rank embryos for transfer or cryopreservation. It supports clinical judgement rather than replacing it. The Istanbul consensus set out shared terminology so that laboratories describe embryos consistently. For clinicians counselling patients, understanding this vocabulary makes laboratory reports easier to interpret and helps explain why a particular embryo was prioritised for transfer.
Grading exists because the number of embryos available usually exceeds the number transferred. Single-embryo transfer, now standard in many settings, places considerable weight on choosing the embryo with the strongest developmental profile. Morphology remains the most widely used non-invasive selection tool, applied at defined developmental windows and recorded against agreed reference criteria rather than personal impression.
Consistency matters as much as the scale itself. Two embryologists should assign comparable grades to the same embryo, and a single observer should be reproducible over time. Laboratory performance indicators set out in the Vienna consensus, alongside good-practice recommendations from bodies such as ESHRE, help audit whether grading and culture conditions meet expected standards, so a grade means the same thing across cycles and staff.
Assessing cleavage-stage embryos
Cleavage-stage assessment is performed on day two and day three, when the embryo divides without appreciable growth in overall volume. Expected development is roughly four cells on day two and around eight cells on day three. Departures from this timing, in either direction, are noted, because both delayed and unusually rapid division can signal reduced developmental potential.
Beyond cell number, several morphological features are recorded and graded against reference descriptions rather than measured precisely, which is one reason calibration between observers is emphasised. Each contributes to an overall impression of developmental quality, and no single feature is read in isolation. The core observations are:
These features combine into a summary category, commonly described as good, fair or poor. The grade informs whether an embryo is transferred at the cleavage stage or cultured onward to the blastocyst stage. Extended culture allows further self-selection, since not every cleavage-stage embryo reaches blastocyst, but it is a laboratory decision weighed against each programme's culture performance.
- Cell number relative to the count expected for that day
- Evenness of blastomere size, or symmetry
- The proportion of cytoplasmic fragmentation
- The presence of multinucleation in one or more blastomeres
Grading the blastocyst
By day five or six, a competent embryo forms a blastocyst, with a fluid-filled cavity, an inner cell mass that will form the fetus, and an outer trophectoderm that gives rise to the placenta. The widely used Gardner-style system, reflected in the Istanbul consensus, describes three elements together, producing a compact grade such as 4AB.
A blastocyst grade combines three separate assessments, recorded individually so the report stays transparent. Reading them together gives a fuller picture than any single letter, and clarifies why, for instance, a well-expanded blastocyst with a sparse inner cell mass is not equivalent to one with an abundant inner cell mass. The three components are:
Each component carries some prognostic weight, and trophectoderm grade in particular has been associated with implantation in several series, though these associations are drawn at population level and do not determine any individual outcome. Timing also matters: a blastocyst reaching full expansion on day five is generally regarded differently from one arriving on day six, reflecting developmental pace.
- Expansion: the degree of blastocoel cavity growth and zona thinning, scored numerically from an early cavity through to a fully hatched blastocyst
- Inner cell mass: graded from A to C by cell number and cohesion, from tightly packed to sparse
- Trophectoderm: graded from A to C by whether it forms a cohesive layer of many cells or a loose layer of few
Morphokinetics and time-lapse imaging
Time-lapse incubation captures images at short intervals, so embryos can be assessed continuously without removal from stable culture conditions. Beyond that convenience, it records the timing of developmental events, or morphokinetics, such as the appearance of specific cell numbers and the onset of blastulation. These timings can be combined into algorithms intended to refine the ranking of embryos.
Reported parameters include the times to two, three and five cells, the duration of individual cell cycles, and the start of blastocoel formation. Deviations from expected intervals, whether accelerated or delayed, have been linked with lower developmental competence in some datasets. Several commercial and in-house models translate these observations into a single selection score.
The honest position is that the evidence remains mixed. Systematic reviews have not shown that time-lapse selection reliably improves live birth compared with conventional assessment, and algorithms often transfer poorly between clinics with different populations and protocols. Time-lapse is a valuable observational and quality-assurance tool; its role as a decisive selection instrument is still being defined.
Where PGT-A fits, and the limits of morphology
Morphology describes appearance, not genetic constitution. Preimplantation genetic testing for aneuploidy (PGT-A) adds ploidy information through a trophectoderm biopsy at the blastocyst stage, analysed by comprehensive chromosome methods. It aims to identify embryos with the expected chromosome complement for transfer, and functions as one input among several rather than a replacement for morphological and clinical assessment.
The correlation between morphology and ploidy is real but weak. Embryos of favourable appearance can be aneuploid, and some blastocysts graded lower still prove euploid. Mosaicism, where a biopsy samples both normal and abnormal cells, further complicates interpretation and calls for careful, non-directive counselling. Testing also carries cost, biopsy, and cryopreservation considerations that belong in a shared decision with the patient.
For clinicians, the practical message is that grading is a probabilistic aid, subject to observer variation and to biology it cannot see. Familiarity with the criteria supports informed conversations with the laboratory and with patients, and helps set realistic expectations. Structured education has a clear place here, provided it is framed accurately and does not overstate what appearance alone can tell us.
A course can build shared vocabulary and interpretive skill, but a certificate of attendance attests participation and learning, not independent competence to grade or biopsy embryos, which develops through supervised laboratory practice and formal embryology qualifications. Organisations such as ATDERA develop faculty-led educational programmes for clinicians with this distinction firmly in mind.