What Happens Inside an IVF Laboratory?
Grace
Healthcare Content Curation Team
A Step-by-Step Walkthrough
From the moment eggs are collected to the day an embryo is transferred — here is exactly what happens inside the IVF laboratory, and what the embryologist does at each stage.
Setting the Scene — The IVF Laboratory
The IVF laboratory is the most technically demanding environment in reproductive medicine. It must replicate, in a controlled artificial setting, the conditions that the fallopian tube and uterus provide naturally. Temperature, pH, gas composition, light levels, and chemical exposure are all tightly managed. Even a brief disruption — a temperature swing of 1°C, an accidental exposure to volatile compounds from cleaning products — can compromise embryo development.
The embryologist is the guardian of this environment. Their decisions from the morning of egg collection through to the day of transfer directly determine the outcome of the cycle. Here is exactly how those six days unfold.
Step 01 — Egg Collection (Oocyte Retrieval)
Egg collection takes place exactly 34 to 36 hours after the trigger shot — timed so that the oocytes have completed final maturation but the follicles have not yet ruptured spontaneously. The procedure is performed in a clinical theatre, not the laboratory, but the embryologist's role begins simultaneously.
Under ultrasound guidance, the clinician passes a fine needle through the vaginal wall into each ovarian follicle and aspirates the follicular fluid. The aspirate is passed immediately through a warm hatch into the adjoining laboratory, where the embryologist searches the fluid under a stereomicroscope for the oocyte. Each cumulus-oocyte complex (COC) — the oocyte surrounded by its cloud of cumulus granulosa cells — is identified, graded, and transferred into pre-equilibrated culture medium in a culture dish. The dish is placed immediately into the incubator. The embryologist calls out each oocyte found to the clinical team, building the count in real time.
Culture dishes, oil overlay, and incubator conditions must be equilibrated overnight before the procedure. Culture medium is protein-supplemented and CO2-buffered to maintain a pH of 7.2 to 7.4 at 37°C. The temperature of the heated stage on the microscope must be verified before each session. Oocytes are never left outside the incubator for more than 60 to 90 seconds.
Step 02 — Sperm Preparation
While egg collection proceeds, a second embryologist (or the same one in smaller labs) prepares the sperm sample. The patient's partner produces a semen sample on the same morning, typically 2 to 3 hours before egg collection. The raw semen contains sperm alongside seminal plasma proteins, dead cells, white blood cells, and debris — none of which should enter the culture dish with the oocyte.
The embryologist processes the sample using one of two techniques. In the swim-up method, motile sperm swim upward into clean medium, leaving debris behind. In Density Gradient Centrifugation (DGC), semen is layered over a density gradient and centrifuged; morphologically normal, motile sperm pellet at the bottom while debris stays above. The resulting washed sperm sample is assessed under the microscope for concentration, motility, and morphology, then diluted to the appropriate concentration for insemination or ICSI.
Sperm preparation media must be pre-warmed and pre-equilibrated. The embryologist documents the raw semen parameters — including volume, motility, concentration, and morphology — before and after preparation. Unusual findings such as very low concentration, complete absence of motile sperm, or a very high white blood cell count are immediately reported to the clinician.
Step 03 — Fertilisation: IVF or ICSI
Approximately 2 to 4 hours after egg collection, the embryologist performs fertilisation. The method depends on the clinical indication.
In standard IVF, a fixed number of prepared, motile sperm — typically 50,000 to 200,000 per mL — are added directly to the culture dish containing the oocyte-cumulus complex. The dish is returned to the incubator and the sperm are left to naturally penetrate the cumulus and fertilise the oocyte overnight.
In ICSI (Intracytoplasmic Sperm Injection), the embryologist first removes the cumulus cells from each oocyte using a fine pipette and brief hyaluronidase enzyme exposure, then assesses oocyte maturity under the microscope. Only MII oocytes — those with a visible first polar body — are eligible. Using a micromanipulator, a precision robotic instrument attached to an inverted microscope, the embryologist selects a single morphologically normal sperm, immobilises it, loads it into a fine glass injection needle approximately 6 to 7 micrometres in diameter, and injects it directly through the zona pellucida into the oocyte cytoplasm. This is performed on every mature oocyte, one by one.
ICSI requires years of training to achieve consistent, proficient technique. Incorrect injection angle risks damage to the meiotic spindle, and too much pressure on injection causes oocyte degeneration. The embryologist assesses the tactile resistance of each oolemma during injection and adjusts accordingly. Workstation temperature maintenance is critical, as the oocyte spindle is temperature-sensitive.
Day 1 — The Fertilisation Check
The morning after fertilisation, 16 to 18 hours later, the embryologist performs the fertilisation check under the inverted microscope.
Normal fertilisation shows exactly 2 pronuclei (2PN) and 2 polar bodies — one copy of each parental genome present, confirming a diploid embryo. Abnormal findings include 1PN (indicating parthenogenesis or failed second polar body extrusion), 3PN (indicating polyspermy or failed polar body extrusion — a triploid embryo that is not usable), and 0PN (indicating failed fertilisation or premature syngamy).
Only 2PN embryos are continued in culture. The fertilisation rate — the number of 2PN embryos divided by the number of mature oocytes injected — is a key laboratory performance indicator and is expected to reach 70 to 80% with ICSI.
Step 04 — Embryo Culture: Days 1 to 5
After the fertilisation check, 2PN embryos are moved into fresh culture medium and placed back in the incubator. For the next 4 to 5 days, the embryologist observes the embryos at defined time points without disturbing them unnecessarily.
On Day 2, at 44 to 48 hours, embryos should be at the 2 to 4 cell stage. The embryologist checks for equal cell size and minimal fragmentation. On Day 3, at 68 to 72 hours, embryos should be at the 6 to 8 cell stage. Compaction — where cells flatten and adhere together — may begin. This is also when embryonic genome activation occurs, meaning the embryo's own DNA switches on for the first time.
On Day 4, at around 96 hours, the 16 to 32 cell morula forms and cells compact tightly into a solid ball. On Days 5 to 6, at 110 to 132 hours, the blastocyst forms. A fluid-filled cavity called the blastocoel develops, and the inner cell mass (which becomes the future embryo) and trophectoderm (which becomes the future placenta) are now distinguishable. This is the optimal stage for embryo transfer or cryopreservation.
Modern IVF labs use time-lapse incubators with built-in cameras that photograph embryos every 5 to 10 minutes without removing them from the controlled environment. This creates a developmental movie of every embryo, providing additional morphokinetic data on the timing of each cell division that correlates with embryo competence.
Step 05 — Embryo Grading
On Day 3 and Day 5, the embryologist grades each embryo according to established morphological criteria.
For Day 3 grading, the embryologist assesses cell number (ideally 6 to 8 cells on Day 3), fragmentation percentage (ideally less than 10%), and cell size symmetry. A score of Grade 1 or Type A indicates 8 equal cells with minimal fragmentation.
For Day 5 blastocyst grading, the Gardner Grading System is used and three parameters are assessed. The expansion grade runs from 1 to 6 and reflects the degree of blastocoel expansion and hatching status — Grade 3 to 4 represents a fully expanded blastocyst, Grade 5 indicates hatching, and Grade 6 indicates a fully hatched blastocyst. The ICM grade runs from A to C — Grade A indicates a tightly packed mass with many cells, Grade B indicates loosely grouped cells, and Grade C indicates very few cells. The trophectoderm grade also runs from A to C — Grade A indicates many cells forming a cohesive layer, Grade B indicates few cells, and Grade C indicates very few large cells.
A 4AA blastocyst — fully expanded with Grade A ICM and Grade A trophectoderm — represents the best possible morphological score. The embryologist selects the highest-graded blastocyst for transfer and prioritises the freeze order for remaining embryos based on grade.
Morphological grading is observer-dependent, and inter-embryologist variability is well documented. AI-assisted grading software is increasingly used to provide objective, reproducible assessments and reduce bias.
Step 06 — Embryo Transfer or Vitrification
On Day 5 or Day 6, the embryologist and clinician decide whether to perform a fresh embryo transfer or to freeze all embryos through vitrification for a future frozen embryo transfer (FET) cycle.
In a fresh transfer, the embryologist loads the selected blastocyst into a fine, soft catheter using a tiny volume of culture medium and hands it to the clinician, who passes it through the cervix into the uterine cavity under ultrasound guidance. The embryologist then checks the catheter under the microscope to confirm the embryo was successfully released. The whole procedure takes 5 to 10 minutes.
In vitrification, remaining good-quality embryos — or all embryos in a freeze-all cycle — are rapidly frozen. This technique plunges embryos from 37°C to −196°C (liquid nitrogen temperature) in milliseconds. The embryologist exposes the embryo to cryoprotectants in a stepwise protocol to replace intracellular water and prevent ice crystal formation, then loads it onto a carrier device such as a Cryotop and plunges it into liquid nitrogen. Vitrified embryos can be stored for years without loss of viability.
During fresh transfer, the embryologist must keep the loaded catheter warm, confirm embryo identity against the patient file, and be present in the theatre to confirm successful embryo delivery. Witnessing protocols — double-checking patient identity at every step — are one of the most important safety procedures in the IVF laboratory.
When Is ICSI Used Instead of Standard IVF?
ICSI is indicated in several specific clinical situations. These include severe male factor infertility with low sperm count, poor motility, or very poor morphology; surgically retrieved sperm (PESA, MESA, TESE) that cannot penetrate the zona naturally; previous total or near-total fertilisation failure with standard IVF; very few oocytes retrieved where the risk of all failing to fertilise naturally is unacceptable; and Preimplantation Genetic Testing (PGT) cycles, where ICSI prevents contamination of the biopsy sample with residual sperm DNA on the zona.
Standard IVF is preferred when sperm parameters are adequate. It is less technically demanding, avoids mechanical stress on the oocyte from injection, and is sufficient for most couples with tubal factor or unexplained infertility.
Quality Control and Patient Safety in the IVF Lab
The IVF laboratory operates under strict quality control standards that are central to every embryologist's daily practice. Two principles above all others govern laboratory safety.
The first is witnessing protocols. Every embryo, sperm sample, and culture dish is labelled with the patient's unique identifier. At every procedural step — insemination, transfer to a new dish, loading for transfer — two embryologists independently verify that the correct patient's samples are being used. In many modern labs, electronic witnessing systems scan barcodes at each step and generate an alert if there is any mismatch. Preventing mix-ups is the single most critical safety function in the IVF laboratory.
The second is environmental control. Temperature at 37°C, CO2 concentration at 5 to 6%, O2 concentration at 5 to 6% in modern labs, and pH between 7.2 and 7.4 must be maintained within narrow ranges at all times. Incubators are checked and logged at least twice daily. Volatile organic compounds from cleaning products, perfumes, and solvents are toxic to embryos, so laboratory staff must not wear fragrance. HEPA and activated carbon air filtration systems are standard. The temperature of heated stages, warm plates, and warming blocks is checked before every procedure.
IVF Laboratory Timeline at a Glance
On Day 0, egg collection and sperm preparation take place. The embryologist identifies and counts oocytes and prepares sperm by DGC or swim-up. Later on Day 0, fertilisation is performed — either inseminating dishes for standard IVF or injecting each MII oocyte for ICSI.
On Day 1, the fertilisation check is carried out. Each embryo is scored as 2PN (normal), 3PN (discard), or 0PN (failed). On Day 2, a cleavage check confirms 2 to 4 cells and checks symmetry and fragmentation. On Day 3, embryos are graded at the 6 to 8 cell stage, and a Day 3 transfer is performed in some protocols.
On Day 4, a morula and compaction check confirms whether cells are compacting normally and identifies any arrested embryos. On Days 5 to 6, blastocysts are graded using the Gardner system, the best blastocyst is selected for transfer, and the remainder are vitrified.
Key Revision Points for Embryology Students
Egg collection occurs 34 to 36 hours after the trigger shot, and the embryologist identifies cumulus-oocyte complexes in the follicular fluid. Sperm preparation by DGC or swim-up removes seminal plasma, debris, and dead cells. In standard IVF insemination, 50,000 to 200,000 motile sperm per mL are added to the COC for natural penetration overnight. In ICSI, a single sperm is injected per MII oocyte using a micromanipulator.
The Day 1 fertilisation check identifies 2PN and 2 polar bodies as normal, 3PN as polyspermy which is discarded, and 0PN as a failed fertilisation. Embryonic genome activation occurs at the 4 to 8 cell stage on Days 2 to 3. The blastocyst forms by Days 5 to 6 with a blastocoel cavity, inner cell mass, and trophectoderm.
Gardner grading assesses expansion (1 to 6), ICM grade (A/B/C), and trophectoderm grade (A/B/C). The best blastocyst grades are 4AA or 5AA. Vitrification is an ultra-rapid freezing process to −196°C where cryoprotectants prevent ice crystal damage. A freeze-all strategy is used to prevent OHSS or when the endometrium is suboptimal for transfer. Witnessing protocols require double-checking patient identity at every single step without exception.
Conclusion
The IVF laboratory is where science and skill converge. Every embryologist who has mastered this process started exactly where you are — reading about it, then watching it, then doing it under supervision, then doing it independently. The six steps described in this blog are the foundation of daily embryology practice.
Disclaimer: Educational content for aspiring embryology students. Reviewed by GRACE faculty. GRACE — Reproductive and Clinical Embryology Institution.https://www.graceembryology.com/
Published by
Grace
Healthcare Content Curation Team
