Bottom line up front: Between egg retrieval and embryo transfer, your embryos spend 3–6 days in the IVF laboratory. What happens during those days — and the quality of the lab handling your embryos — has an enormous impact on your outcome. This guide explains the process so you understand what the embryologist is doing, what the grading numbers mean, and how to evaluate a Colombian lab's capability.
Day 0: Retrieval Day
After egg retrieval, the embryologist examines each follicular fluid sample under the microscope to identify and isolate the eggs. Not every follicle contains a mature egg. Typically, 75–85% of retrieved eggs are mature (MII stage) and suitable for fertilisation. Immature eggs (MI or GV stage) are usually not usable.
Fertilisation: Conventional vs ICSI
Conventional insemination: Approximately 50,000–100,000 motile sperm are placed around each egg in a culture dish. Sperm compete to penetrate the egg naturally. Best suited for cases with normal sperm parameters.
ICSI: A single selected sperm is injected directly into each mature egg using a microscopic needle. Required for male factor infertility, low sperm count, previous fertilisation failure, or when using frozen sperm. Many Colombian clinics now use ICSI for all cases due to higher and more consistent fertilisation rates (70–80% vs 60–70% for conventional).
Fertilisation is assessed 16–18 hours later. A normally fertilised egg shows two pronuclei (one from each parent). Eggs with zero or more than two pronuclei are discarded.
Days 1–5: Embryo Development
| Day | Stage | What Happens | What Embryologists Assess |
|---|---|---|---|
| Day 1 | Zygote (1 cell) | Pronuclei visible → confirm fertilisation | Number of pronuclei (2PN = normal) |
| Day 2 | 2–4 cells | First cell divisions | Even cell division, minimal fragmentation |
| Day 3 | 6–8 cells | Continued division, embryonic genome activates | Cell number, symmetry, fragmentation (Grade 1–4) |
| Day 4 | Morula | Cells compact into a ball | Compaction quality |
| Day 5 | Blastocyst | Fluid-filled cavity forms, inner cell mass differentiates | Expansion, ICM quality, trophectoderm quality |
Understanding Blastocyst Grading
A Day 5 blastocyst grade like "4AA" has three components:
- Number (1–6): Expansion stage. 1 = early, 3 = full, 4 = expanded, 5–6 = hatching/hatched. Higher is more developed
- First letter (A–C): Inner cell mass (ICM) quality — this becomes the baby. A = many tightly packed cells, B = fewer or loosely grouped, C = very few cells
- Second letter (A–C): Trophectoderm quality — this becomes the placenta. A = many cells forming a cohesive layer, B = fewer cells, C = very few cells
4AA = fully expanded blastocyst with excellent ICM and trophectoderm — the highest quality grade. 3BB = full blastocyst with good (not excellent) ICM and trophectoderm — still a very transferable embryo. 3CC = lower quality but can still result in a healthy pregnancy.
A 3BB embryo can produce a perfectly healthy baby. Grading predicts implantation probability, not baby quality. Do not be discouraged by B or C grades — they reduce the statistical odds slightly but do not determine the outcome.
How to Assess a Colombian Lab's Quality
- Blastocyst rate: What percentage of fertilised eggs reach blastocyst stage? Good labs achieve 40–60%. Below 30% is concerning
- Fertilisation rate: 70–80% with ICSI is standard. Below 60% suggests technical issues
- Vitrification survival: >95% of embryos should survive freeze-thaw. Below 90% indicates suboptimal technique
- Equipment: Look for time-lapse incubators (EmbryoScope), HEPA-filtered air, VOC-controlled environment, and backup power systems
- Embryologist credentials: Ask about the senior embryologist's training, years of experience, and whether they hold international certifications
Want to Know About Lab Quality?
We evaluate Colombian clinic labs on fertilisation rates, blastocyst rates, and equipment before making referrals. Ask us about any clinic.
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