From Fertilised Egg to Embryo
After successful fertilisation, embryos are carefully cultured in our laboratory under conditions that closely mirror the natural environment of the human fallopian tube and uterus. Over the next 3–6 days, embryologists monitor each embryo’s development, assessing cell division, morphology, and overall quality at key milestones to select the best candidate(s) for transfer.
Stages of Embryo Development
Day 1: Fertilisation Confirmation (Zygote)
At 16–18 hours post-insemination, the embryologist confirms normal fertilisation by the presence of two pronuclei (2PN). The fertilised egg is now called a zygote. Abnormally fertilised eggs are identified and removed from culture.
Day 2–3: Cleavage Stage
The zygote begins dividing. By Day 2, a normally developing embryo has 2–4 cells; by Day 3, it should have 6–8 cells. The embryologist evaluates each embryo based on the number and symmetry of cells, the rate of cell division, and the degree of fragmentation (small cellular debris between cells). Low fragmentation and even cell division indicate higher embryo quality.
Day 4: Morula Stage
By Day 4, the embryo compacts into a morula — a tight ball of 16–32 cells. Cell boundaries become less distinct as the cells begin communicating and working together. This stage marks the embryo’s transition from individual cells to an organised structure.
Day 5–6: Blastocyst Stage
The blastocyst is the most advanced stage of pre-implantation embryo development. It consists of 100–200 cells organised into two distinct groups: the inner cell mass (ICM), which will develop into the foetus, and the trophectoderm (TE), which will form the placenta. A fluid-filled cavity (blastocoel) expands within the embryo.
Blastocysts are graded using a standardised system that evaluates the degree of expansion, the quality of the inner cell mass, and the quality of the trophectoderm (e.g., a grade of 4AA represents a fully expanded blastocyst with a tightly packed ICM and a cohesive TE — considered excellent quality).
Why Culture to Blastocyst Stage?
Extended culture to Day 5–6 offers several clinical advantages. It provides a natural selection process — only the strongest embryos survive to the blastocyst stage, meaning those that do are more likely to implant. Blastocyst transfer is more physiologically aligned with natural conception, as embryos naturally reach the uterus at this stage. It also improves the accuracy of embryo selection, supports single embryo transfer strategies (reducing multiple pregnancy risk), and is required before preimplantation genetic testing (PGT).
Not all embryos reach the blastocyst stage. On average, approximately 40–60% of fertilised eggs develop to a usable blastocyst. While this natural attrition can feel discouraging, it reflects the biological reality that not every embryo has the developmental potential to establish a pregnancy.
Time-Lapse Monitoring (EmbryoScope)
Our laboratory uses time-lapse incubation technology (such as the EmbryoScope) to monitor embryo development continuously without removing embryos from the controlled incubator environment. A built-in camera captures images of each embryo every 10–20 minutes, creating a detailed time-lapse video of development from fertilisation to blastocyst.
This technology offers two key benefits: it provides uninterrupted, stable culture conditions (since the incubator door does not need to be opened for manual checks), and it gives embryologists a far more detailed developmental picture than conventional daily assessments alone. The time-lapse data helps identify embryos with the best implantation potential based on precise developmental kinetics.
Preimplantation Genetic Testing (PGT)
PGT-A (Aneuploidy Screening)
PGT-A screens embryos for chromosomal abnormalities (aneuploidy) — the most common cause of implantation failure, miscarriage, and certain genetic conditions such as Down syndrome. A small biopsy of 5–10 trophectoderm cells is taken from the blastocyst on Day 5 or 6 and sent for genetic analysis. The embryo is cryopreserved while awaiting results, which typically take 1–2 weeks.
PGT-M (Monogenic Disorder Testing)
PGT-M is used when one or both partners are known carriers of a specific single-gene disorder — such as cystic fibrosis, sickle cell disease, thalassemia, or spinal muscular atrophy. The test identifies embryos that are unaffected by the condition, allowing only healthy embryos to be transferred.
PGT-SR (Structural Rearrangements)
PGT-SR is recommended for patients with known chromosomal structural rearrangements (such as translocations or inversions), which increase the risk of miscarriage or chromosomally unbalanced pregnancies. Testing identifies embryos with a balanced or normal chromosomal complement.
Frequently Asked Questions
On average, 40–60% of normally fertilised eggs develop into usable blastocysts by Day 5–6. For example, if 8 eggs fertilise normally, you might expect 3–5 blastocysts. This varies by age, egg quality, and sperm quality.
In most modern IVF programmes, Day 5 (blastocyst) transfer is preferred because it allows better embryo selection, higher implantation rates per embryo, and supports single embryo transfer — reducing the risk of twins or triplets. Day 3 transfer may be considered if very few embryos are available or in specific clinical scenarios where extended culture is not advisable.
Embryos that arrest (stop developing) during culture are not viable for transfer or freezing. Embryo arrest is a natural biological process and reflects that the embryo did not have sufficient developmental competence. It is not caused by laboratory error.
PGT-A is generally recommended for patients aged 35 and above, those with recurrent miscarriage or implantation failure, and patients who wish to maximise the chance of selecting a chromosomally normal embryo. Your specialist will discuss whether PGT-A is appropriate for your individual situation, including the benefits, limitations, and cost implications.
Trophectoderm biopsy is considered safe when performed by experienced embryologists. The biopsy removes a small number of cells from the outer layer (future placenta), not from the inner cell mass (future foetus). Large-scale studies show no increased risk of birth defects or developmental issues in children born from biopsied embryos.
This is a difficult but possible outcome, particularly for older patients. If no euploid (chromosomally normal) embryos are available, your specialist will discuss options including another IVF cycle, transfer of a mosaic embryo (in selected cases), or alternative pathways such as donor eggs.
Yes. Many clinics, including ours, offer patients the opportunity to view the time-lapse development footage of their embryos. This can be shared during your embryo transfer consultation or provided digitally.