What Happens in the IVF Lab: From Egg to Embryo

Bottom line up front: The IVF laboratory is where the science happens. After your eggs are retrieved and sperm is collected, embryologists take over — fertilising eggs, culturing embryos for 5–6 days, grading their quality, optionally testing their genetics, and preparing them for transfer or freezing. Understanding what happens at each stage helps you interpret your clinic's daily updates and ask better questions.

Day 0: Egg Retrieval and Sperm Collection

Egg retrieval is performed under light sedation. Using ultrasound guidance, the doctor inserts a thin needle through the vaginal wall into each ovarian follicle and aspirates (suctions) the fluid, which contains the egg. The procedure takes 15–30 minutes.

The aspirated fluid is immediately passed to the embryology lab, where embryologists examine it under a microscope to identify and isolate the eggs. Not every follicle contains a mature egg — typically 70–80% of retrieved eggs are mature (MII stage) and suitable for fertilisation.

Sperm is collected the same day (or thawed, if previously frozen). The sample is processed through a technique called "sperm washing" or density gradient centrifugation, which separates the healthiest, most motile sperm from the rest of the sample.

Day 0–1: Fertilisation

Conventional IVF

Each mature egg is placed in a dish with approximately 50,000–100,000 prepared sperm. The sperm must penetrate the egg's outer layer (zona pellucida) on their own. Fertilisation is checked 16–18 hours later by looking for two pronuclei (one from the egg, one from the sperm) — confirmation that fertilisation has occurred normally.

ICSI (Intracytoplasmic Sperm Injection)

A single sperm is selected and injected directly into the egg using a microscopic needle. ICSI is used when sperm quality is poor (low count, poor motility, abnormal morphology), when previous conventional IVF showed poor fertilisation, when using frozen sperm or surgically retrieved sperm, or when PGT-A is planned (to avoid contamination from extra sperm DNA on the egg surface).

ICSI fertilisation rates are typically 70–85% for mature eggs. Most Colombian clinics include ICSI as standard in their IVF pricing — it is not always a separate add-on as it often is in the US.

Days 1–3: Cleavage Stage

Successfully fertilised eggs begin dividing. By day 2, a healthy embryo should have 2–4 cells. By day 3, it should have 6–8 cells. Embryologists assess cell number, symmetry, and the degree of fragmentation (bits of cellular debris between the cells). Low fragmentation and even cell division are positive signs.

Some clinics transfer embryos at day 3 (cleavage stage). However, the trend in modern IVF is to culture embryos to day 5 (blastocyst stage), which provides better selection information and higher per-transfer success rates.

Days 3–5: The Blastocyst Transition

Between days 3 and 5, the embryo undergoes a critical transition. It activates its own genome (until this point, it runs on maternal RNA from the egg), compacts into a dense ball called a morula (day 4), and then forms a blastocyst (day 5–6) — a fluid-filled structure with two distinct cell types: the inner cell mass (ICM, which becomes the baby) and the trophectoderm (which becomes the placenta).

This transition is a major quality checkpoint. Many embryos that looked acceptable at day 3 will arrest (stop developing) during this phase. The ones that reach blastocyst stage have demonstrated a fundamental level of viability.

Embryo Grading

Blastocysts are graded on three criteria:

A grade like "4AA" means a well-expanded blastocyst with excellent ICM and excellent trophectoderm — this is a top-quality embryo. A "3BB" is good. A "3BC" or "4CB" is lower quality but can still result in a healthy pregnancy. Grading is somewhat subjective and varies between embryologists, so do not panic over a grade that seems average — many healthy babies come from "B" and "C" graded embryos.

PGT-A: Genetic Testing

Pre-implantation Genetic Testing for Aneuploidies (PGT-A) involves removing 5–10 cells from the trophectoderm (future placenta) of a blastocyst and sending them for genetic analysis. The biopsy does not harm the embryo — it recovers during the freezing and thawing process.

PGT-A identifies embryos with the correct number of chromosomes (euploid) versus those with too many or too few (aneuploid). Aneuploid embryos are the primary cause of implantation failure, miscarriage, and certain genetic conditions. Transferring only euploid embryos significantly improves per-transfer success rates and reduces miscarriage risk.

Vitrification: Freezing Embryos

Modern embryo freezing uses vitrification — an ultra-rapid freezing technique that prevents ice crystal formation and achieves survival rates of 95–99% upon thawing. Vitrified embryos can be stored indefinitely with no degradation in quality. This technology is why freeze-all strategies have become so common — there is no longer a meaningful disadvantage to freezing compared to fresh transfer.

The Bottom Line

The IVF lab is where biology meets technology. Understanding the process — from retrieval through fertilisation, culture, grading, and testing — helps you make sense of the daily updates you receive from your clinic and prepares you for the natural attrition that occurs at each stage. Colombian clinics use the same laboratory techniques and equipment as top-tier US and European clinics, including ICSI, extended blastocyst culture, PGT-A, and vitrification.

Read more: Single vs Double Embryo Transfer | IVF Glossary | How Many Cycles?