Most patients never see the IVF lab. You hand over a sperm sample or have your eggs collected, and then you wait for phone calls. Yet the lab is where your eggs, sperm and embryos spend nearly all their time outside the body, and where many of the decisions that shape your cycle are made. This guide follows a typical cycle through the lab, step by step, then covers how labs keep embryos safe and what is reasonable to ask about.
Who works in the lab
An IVF laboratory usually has two linked areas. The andrology side deals with sperm: assessment, preparation and freezing. The embryology side handles eggs and embryos from retrieval to transfer or freezing. Embryologists are scientists rather than doctors. They often speak to patients directly about fertilization and embryo progress, and they work closely with the physician who performs the retrieval and transfer.
A cycle through the lab
Retrieval day: finding the eggs
During retrieval, fluid from each follicle is passed straight to the lab. An embryologist scans it under a microscope to find each egg, which is still wrapped in its cloud of cumulus cells. The eggs are counted, rinsed and placed in culture medium in an incubator. The count you hear in the recovery room comes from this step.
Preparing the sperm
A fresh or frozen sample is processed to separate moving sperm from seminal fluid and debris. The method depends on sample quality. Our guide to sperm preparation covers the techniques.
Insemination
With conventional IVF, prepared sperm are added to each egg's dish and fertilization happens naturally. With ICSI, the cumulus cells are first stripped away, the mature eggs are identified, and an embryologist injects a single sperm into each mature egg using a fine glass needle. The choice depends on sperm quality, previous fertilization results and clinic policy. See ICSI explained.
The fertilization check
The next morning, embryologists check each egg for signs of normal fertilization: a pair of pronuclei carrying the maternal and paternal genetic material. The updated ESHRE/ALPHA Istanbul Consensus recommends this check at 16 to 17 hours after insemination, and keeps two pronuclei (2PN) as the preferred finding (ESHRE, 2025). Your "fertilization report" call usually comes after this check.
Culture: days 2 to 6 (sometimes 7)
Embryos are kept in incubators that control temperature and gas levels and keep the culture medium stable. Some labs check them at set times under a microscope. Others use time-lapse incubators that photograph them continuously. Embryos are assessed at the cleavage stage and, if culture continues, at the blastocyst stage. For a day-by-day picture of development, see embryo development from day 1 to 6.
Biopsy, if you are having genetic testing
For PGT, a few cells are removed from the outer layer (trophectoderm) of a blastocyst and sent to a genetics lab. ASRM notes that biopsy at this stage has less impact on implantation than the older practice of removing a cell on day 3, though its effects are not fully understood (ASRM, 2024).
Transfer or freezing
On transfer day, the embryologist loads the chosen embryo into a thin catheter and hands it to the doctor. Embryos that are not transferred, or all embryos in a freeze-all cycle, are vitrified (frozen very rapidly) and stored in liquid nitrogen.
Freezing has become central
Vitrification turns the embryo's contents into a glass-like state almost instantly, avoiding the damaging ice crystals of older slow-freezing methods. Frozen transfers have become a major part of IVF. In the UK they rose from 24% of IVF cycles in 2014 to 48% in 2024, and there were 10,450 embryo-freezing cycles in 2024 (HFEA, 2026). In practice, how well a lab freezes and warms embryos matters to most patients, not only those choosing to freeze. If you plan to store embryos abroad, our guide to embryo freezing logistics abroad covers storage contracts and fees.
Technology: useful tools versus "add-ons"
Labs differ in the equipment they use. Some of it is standard good practice. Some is marketed as a way to raise success rates. It helps to know which is which.
| Technology | What it does | What the UK regulator says about improving birth rates |
|---|---|---|
| Time-lapse imaging and incubation | Continuous photos of embryos without removing them from the incubator | Rated black: moderate to high-quality evidence shows no effect on the chance of a baby |
| Embryo "glue" (hyaluronate-enriched transfer medium) | A special medium used at transfer | Rated yellow: conflicting evidence |
| Assisted hatching | Thinning or opening the embryo's shell before transfer | Rated grey: not enough good evidence to rate |
| IMSI | High-magnification sperm selection for ICSI | Rated grey |
Ratings are from the HFEA's add-ons list (last reviewed 2026), which currently rates no add-on green. A lab can still have good reasons to use time-lapse incubators, such as undisturbed culture or a richer record of development. Just do not assume an extra fee for one buys a higher chance of a baby.
How labs keep embryos safe
Much of a good lab's work is invisible: steady incubator conditions, routine equipment checks, careful labelling, and procedures for checking that the right eggs, sperm and embryos are matched at every step. Many labs use double-checking by a second person or electronic tracking at each handling step. Storage tanks need alarms and continuous monitoring. These systems rarely appear on a clinic's website, but a well-run lab should be happy to describe them.
The numbers that reflect lab performance
Your own cycle report contains several lab-sensitive numbers: how many mature eggs fertilized, how many embryos reached blastocyst, how many survived warming. Clinics also track these internally as quality indicators. When a clinic quotes a rate, ask what it was divided by. A blastocyst rate per fertilized egg is not the same as one per egg retrieved. Ask, too, whether it covers patients like you. One cycle's results always reflect your eggs and sperm as well as the lab, so a single disappointing number is not proof of a lab problem.
Embryos you do not use straight away
Stored embryos stay in the lab's care, sometimes for years. Before freezing, you will sign consent forms covering how long they can be kept and what should happen to them if you no longer need them, separate, or cannot be contacted. Those choices are made in the lab's paperwork as much as in the clinic room. Keep your own copy, and update the clinic if your address changes, because storage renewals are usually handled by post or email.
Questions to ask about a clinic's lab
- Is the lab on site, and who runs it? How experienced is the embryology team?
- Will I get a written report showing eggs retrieved, mature eggs, fertilization and embryo development by day?
- How do you decide between conventional IVF and ICSI for me?
- How often do you check embryos, and do you use time-lapse? Is there an extra charge?
- What are your embryo survival rates after warming, and how is that measured?
- How are samples identified and witnessed at each step?
- How are storage tanks monitored, and what happens if an alarm goes off out of hours?
- Can I speak with an embryologist if I have questions about my embryos?
Frequently asked questions
Will I get to talk to the embryologist?
At many clinics, yes. Embryologists often make the fertilization and embryo-progress calls themselves. If yours does not, it is reasonable to ask for a conversation, especially after an unexpected result.
Why do I get daily calls after retrieval?
Each call follows a lab checkpoint: fertilization the next morning, then embryo development at the cleavage and blastocyst stages. Clinics differ in how often they update patients.
Is time-lapse imaging worth paying extra for?
The HFEA rates time-lapse imaging and incubation as having no effect on the chance of a baby, based on moderate to high-quality evidence. A clinic may still use it for other reasons, but it is not a proven way to raise success.
How long can embryos stay frozen?
Storage limits depend on local law and your consent forms, and vary by country. Check the legal limit and the clinic's renewal process before you freeze.
Does the lab decide which embryo is transferred?
The embryology team usually ranks embryos by development and grade, and by genetic result if tested, then discusses the choice with your doctor and you. You should be told which embryo is being transferred and why.
Sources
- ESHRE Focus on Reproduction — Istanbul Consensus update (2025)
- HFEA — Treatment add-ons with limited evidence (reviewed 2026)
- HFEA — Fertility treatment 2024: trends and figures (2026)
- ASRM — The use of PGT-A (2024)
The Takeaway
The lab is where fertilization, embryo growth and freezing happen. Judge it on clear reporting, careful identification and freezing performance, not on how many extra technologies are on the price list.