What actually happens in the IVF lab

Most of IVF happens where you can't see it: a quiet, warm laboratory next to the operating room, run by embryologists. This page walks through what they do with your eggs, sperm and embryos, day by day. Your clinic's exact steps and timings may differ a little — that's normal — but the shape is the same everywhere.

Day 0 — egg collection and the sperm sample

Your doctor collects fluid from the follicles in your ovaries under ultrasound guidance. Each tube of fluid goes straight through a hatch into the lab, where an embryologist searches it under a microscope for eggs. Eggs are tiny — about a tenth of a millimetre — and each one is wrapped in a fluffy cloud of cells that makes it easier to find.

Not every follicle contains an egg, and not every egg is mature. That is why the number of eggs collected is often less than the number of follicles counted on your scans, and why the number that can be used is smaller again. More on why the numbers change →

Meanwhile the sperm sample (from your partner or a donor) is prepared: washed and concentrated so that the healthiest, best-moving sperm are selected.

Day 0, later — insemination

There are two ways to bring egg and sperm together:

• Conventional IVF — thousands of prepared sperm are placed with each egg in a small drop of culture fluid, and one finds its own way in.
• ICSI — a single sperm is picked up and injected directly into each mature egg with a very fine glass needle. What ICSI is and when it's used →

Which one your clinic uses depends mainly on the sperm sample and your history. Both are routine.

Day 1 — checking fertilisation

Around 16–18 hours later the embryologist looks at each egg for the signs that fertilisation happened normally: two small circles inside the egg (the genetic material from the egg and the sperm) called pronuclei. An egg with two is a normally fertilised egg — a zygote. Eggs with none, one, or three are not used, because they will not develop into a healthy embryo.

Not all mature eggs fertilise. Around 65–80% is typical, and it varies from person to person and cycle to cycle. This is the first number your clinic will call you with.

Days 2–3 — the embryo divides

The fertilised egg starts to divide: two cells, then four, then eight by day 3. The embryologist checks progress and notes how many cells there are and how even and clean the cells look. This is where embryo grading starts. What your embryo grade means →

Some clinics transfer or freeze embryos on day 3. Many now wait longer.

Days 5–6 — the blastocyst

By day 5 a well-developing embryo has become a blastocyst: a hollow ball of a hundred or so cells with a fluid-filled centre, an outer layer that will become the placenta, and an inner clump that will become the baby. Roughly half of good-quality fertilised eggs get this far — it is normal for the numbers to fall along the way, because the embryos that stop are usually the ones that could not have continued.

Blastocysts are what most clinics transfer, freeze, or biopsy for genetic testing.

Transfer, freezing, or both

• Fresh transfer — one (sometimes two) embryo is loaded into a soft catheter and placed in the uterus, usually on day 3 or day 5. It takes a few minutes and doesn't need anaesthesia.
• Freezing (vitrification) — extra good-quality embryos are frozen in seconds in liquid nitrogen and stored for later. Survival on thawing is very high with modern methods. How embryo freezing works →
• Freeze-all — sometimes the doctor recommends freezing everything and transferring in a later, calmer cycle. This is common and not a bad sign.

What the lab is doing when you're not looking

Between the moments above, embryos live in incubators that hold them at body temperature in air adjusted to match the womb, in tiny drops of culture fluid under a layer of oil. Embryologists check identity at every step — two people, or a barcode system, confirm that the right eggs, sperm and embryos are together every time anything is moved. That "witnessing" is one of the most important things a lab does, and you will never see it.