Fertility

The Days Between Retrieval and Transfer in the Lab

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The days between egg retrieval and transfer can feel like a black box — you hand over eggs and wait for phone calls. Inside the lab, a predictable sequence unfolds: fertilization, a fertilization check, cleavage, and for some embryos the blastocyst stage. Here is what happens each day, why embryos drop out along the way, and which lab add-ons are worth questioning.

Last updated: July 2026

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What happens to your embryos in the lab?

After retrieval, the eggs move to the embryology lab, where the next five to six days follow a set rhythm. On day 0 each mature egg is fertilized. On day 1 the team checks which eggs fertilized normally. Across days 2 and 3 the embryos divide into more cells, and by days 5 to 6 the strongest have reached the blastocyst stage, ready to transfer or freeze.

At every step, some embryos stop developing. This attrition is normal biology, not a lab error, and it is why the number of blastocysts is almost always smaller than the number of eggs retrieved. The lab keeps embryos in tightly controlled incubators that mimic the body's conditions, and updates you along the way. Understanding what the embryo lab culture is doing each day turns a stressful wait into a sequence you can actually follow.

Day 0: fertilization

On the day of retrieval, each mature egg is fertilized in one of two ways. In conventional insemination, eggs and prepared sperm are placed together in a dish and a sperm fertilizes the egg on its own. In ICSI, an embryologist injects a single sperm directly into each egg. ICSI (intracytoplasmic sperm injection) is the direct injection of one sperm into one egg. ICSI was developed for male-factor infertility, when sperm count or movement is too low for conventional fertilization.

Here the evidence matters. For couples without male-factor infertility or a prior fertilization failure, routinely using ICSI on all eggs does not improve live-birth rates over conventional insemination 1. It is an appropriate tool for specific reasons — male factor, previous failed fertilization, or planned genetic testing — rather than a default upgrade that is better for everyone 1. If a clinic recommends ICSI, it is reasonable to ask which of those reasons applies to you.

Day 1: the fertilization check

The morning after fertilization, the embryologist checks each egg for signs that it fertilized normally. A normally fertilized egg shows two pronuclei — one carrying genetic material from the egg, one from the sperm. Eggs that show none, or an abnormal number, did not fertilize correctly and are not used, because they will not develop into a healthy embryo.

This is the first place the numbers narrow. Not every mature egg fertilizes, and not every fertilized egg does so normally, so the count of normally fertilized eggs is usually lower than the number retrieved. The lab typically calls with this fertilization report on day 1. It can be an anxious call, but a drop here is expected and built into how clinicians plan the cycle. The day-1 report simply sets the starting point for the days that follow.

Days 2-3: the cleavage stage

Over the next two days the embryos divide — from two cells to four, to roughly six or eight by day 3. This is the cleavage stage, and it is the point at which a day 3 transfer would happen if that is the plan. Embryologists begin grading here, looking at the number of cells and how even and fragment-free they appear.

Grading is a useful guide, not a verdict. How are embryos graded is a common question, and the honest answer is that grades describe appearance and correlate only loosely with potential, without predicting any single embryo's outcome. For patients with only a few embryos, the team may transfer on day 3 rather than risk two more days of extended culture. For others, the embryos stay in the incubator to see which reach the blastocyst stage, which allows better selection. Either path is legitimate, and which one fits depends on how your embryos are actually growing.

Days 5-6: the blastocyst stage

By days 5 and 6, the embryos that keep developing form blastocysts — more organized structures with a fluid-filled cavity and two distinct cell groups, one that becomes the baby and one that becomes the placenta. Reaching this stage is a meaningful milestone, and many embryos do not make it. That drop-off is the egg to blastocyst attrition funnel, and it is why the number of usable embryos per cycle is often far smaller than the egg count.

Blastocyst culture lets the lab compare embryos and choose the strongest one for transfer. This is what makes elective single-embryo transfer practical: guidance recommends transferring one embryo — often a single blastocyst — for most patients, because it keeps pregnancy rates close to a double transfer while sharply lowering the chance of twins 2. The blastocysts you are not transferring can be frozen for later. How many embryos from one ivf cycle you end up with is one of the most useful numbers to ask your lab for.

Lab add-ons: time-lapse, hatching, and genetic testing

Several extras can be offered during the culture days, and they are worth questioning. Time-lapse incubators photograph embryos continuously so they can be watched without being disturbed; assisted hatching thins the embryo's outer shell; and PGT-A biopsies a few cells from a blastocyst to screen its chromosomes. The UK regulator rates many of these add-ons on a five-tier evidence scale, and several sit in the tiers where good evidence of benefit is lacking 3.

Genetic testing is the add-on people ask about most. Current professional guidance is that the value of PGT-A as a routine screen for all IVF patients has not been demonstrated, and recent multicenter trials found similar overall pregnancy outcomes with and without it 4. It has real uses — recurrent loss, certain ages, or specific goals — but it is not a guaranteed upgrade, and the pgt-a cost is added per embryo tested. Asking a clinic which add-ons it recommends, and what evidence supports each, is a fair and useful conversation to have before you agree.

Fresh transfer or freeze-all?

At the end of culture, your team decides whether to transfer a fresh embryo in this cycle or freeze all the embryos for a later transfer. For most patients this is not a matter of one being clearly better: large randomized trials found no routine live-birth advantage to freezing all embryos over a fresh transfer 5. So freeze-all is a strategy, not an automatic upgrade.

There are specific reasons to freeze everything. Chief among them is avoiding a fresh transfer when the risk of ovarian hyperstimulation syndrome is high, since a fresh-cycle pregnancy can worsen it — freezing lets the body recover first 6. Frozen embryos are stored in the lab, which carries ongoing embryo storage fees, and banking embryos over several retrievals is sometimes planned for people with a low egg yield. Whatever the plan, the embryo transfer procedure itself comes later, once the lab work is done and the timing is right.

Common questions

Usually five to six days for a blastocyst-stage transfer, or three days for a cleavage-stage transfer. During that time the embryos are fertilized, checked, and grown in controlled incubators while the team tracks which keep developing. If all the embryos are frozen, they can be stored much longer and transferred in a later cycle, weeks or months afterward.

This drop-off is normal biology, not a lab mistake. Not every egg fertilizes, not every fertilized egg divides well, and many embryos naturally stop developing before the blastocyst stage. The result is that the number of usable blastocysts is often far smaller than the number of eggs retrieved. Your age, egg and sperm quality, and chance all play a part in how many make it.

Not necessarily. ICSI — injecting a single sperm into each egg — was developed for male-factor infertility and is appropriate for low sperm count or movement, prior fertilization failure, or planned genetic testing. For couples without those factors, using ICSI on all eggs does not improve live-birth rates over conventional insemination. It is reasonable to ask which specific reason applies before agreeing to it.

Often not, based on current evidence. A UK regulator rates many IVF add-ons on a five-tier scale, and several lack good evidence of benefit. Genetic testing of embryos has real uses but has not been shown to help all patients routinely. The useful move is to ask a clinic which add-ons it recommends for you and what evidence supports each, rather than adding them by default.

It depends on your cycle. Large trials show no routine live-birth advantage of freezing all embryos over a fresh transfer, so neither is automatically better. Clinics often freeze everything when the risk of ovarian hyperstimulation is high, letting the body recover before a later transfer. Your team will explain their plan based on how you responded to stimulation and how your embryos developed.

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When to call your clinic during the lab wait

  • —Worsening abdominal bloating or swelling in the days after retrieval, with clothes feeling suddenly tight
  • —Shortness of breath, or a sharp drop in how much you are urinating — possible ovarian hyperstimulation syndrome
  • —Fever, or severe or one-sided pelvic pain that is getting worse rather than better
  • —Heavy vaginal bleeding after the retrieval

For severe bloating with breathlessness, a sharp fall in urination, severe pelvic pain, or heavy bleeding, call your clinic's 24-hour line right away, and go to the emergency room or call 911 if symptoms are severe or you cannot reach the clinic.

This article explains what happens to embryos in the lab. It is educational and not medical advice; your clinic and embryology team can tell you what is happening with your specific embryos and cycle.

References

  1. 1.Practice Committees of ASRM and SART (2026). Intracytoplasmic sperm injection for nonmale factor indications: a committee opinion. American Society for Reproductive Medicine (Fertility and Sterility). linkAbsent male factor or a prior fertilization failure, routinely using ICSI on all oocytes does not improve live-birth rates; ICSI is justified by male factor, prior fertilization failure, or planned genetic testing rather than as a default upgrade.
  2. 2.Practice Committees of ASRM and SART (2021). Guidance on the limits to the number of embryos to transfer: a committee opinion. American Society for Reproductive Medicine (Fertility and Sterility). linkGuidance recommends elective single-embryo transfer — often a single blastocyst — for most patients, keeping pregnancy rates close to a multiple-embryo transfer while sharply reducing twin risk.
  3. 3.Human Fertilisation and Embryology Authority (2024). Treatment add-ons with limited evidence. Human Fertilisation and Embryology Authority (UK). link ✓Many IVF lab add-ons, including time-lapse imaging, assisted hatching, and PGT-A, are rated on a five-tier evidence scale, and several lack good evidence of benefit.
  4. 4.Practice Committees of ASRM and SART (2024). The use of preimplantation genetic testing for aneuploidy: a committee opinion. American Society for Reproductive Medicine (Fertility and Sterility). PMID 38762806 ✓The value of PGT-A as a routine screen for all IVF patients has not been demonstrated, and recent multicenter trials found similar overall pregnancy outcomes with and without it.
  5. 5.Vuong LN, et al. (2018). IVF Transfer of Fresh or Frozen Embryos in Women without Polycystic Ovaries. New England Journal of Medicine. doi:10.1056/NEJMoa1703768 ✓A randomized trial found no significant difference in live birth between frozen and fresh embryo transfer, so routine freeze-all offers no live-birth advantage over a fresh transfer in most patients.
  6. 6.Practice Committee of ASRM (2023). Prevention of moderate and severe ovarian hyperstimulation syndrome: a guideline. American Society for Reproductive Medicine (Fertility and Sterility). linkFreezing all embryos to avoid a fresh transfer is an evidence-based way to reduce the risk of moderate-to-severe ovarian hyperstimulation syndrome.

6 sources, numbered by first appearance. General health information, not medical advice. AI-assisted editorial content — citations link their sources. Editorial policy

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