Key takeaways

  • IVF embryos are watched day by day in the lab: fertilisation check on day 1, cleavage stage on days 2–3, morula on day 4, and blastocyst on days 5–6.
  • Embryo attrition is normal biology — only about 30–60% of fertilised eggs reach the day-5 blastocyst stage even in younger women, so a falling number is expected, not a clinic failure.
  • Blastocysts are graded on the Gardner scale (expansion 1–6, inner cell mass A–C, trophectoderm A–C); higher grades implant more often but never guarantee a baby, and lower grades still make healthy babies.
  • Day-5 blastocyst transfer is preferred when enough good embryos exist because it allows better selection and single embryo transfer; day-3 transfer is reasonable when only 1–3 embryos are available.
  • Add-ons like time-lapse imaging (EmbryoScope) and PGT-A genetic testing help selection in specific situations but are optional — discuss whether they suit your case before paying extra.

Day 0: Egg retrieval and insemination

Day 0 is the day your eggs are collected and meet the sperm. Eggs are retrieved by transvaginal ultrasound-guided aspiration during a 20–30 minute outpatient procedure under sedation — the full experience is covered in our guide to egg retrieval in IVF. The follicular fluid is passed through a hatch into the embryology lab, where embryologists search it under a microscope for each cumulus-oocyte complex (the egg wrapped in its supporting cells).

Eggs are graded for maturity. Metaphase II (M2) eggs are mature and ready to fertilise; metaphase I (M1) eggs are intermediate and may mature in culture; germinal vesicle (GV) eggs are immature and usually not used. Typically 70–80% of retrieved eggs are mature M2, though this varies with trigger timing and individual response. Mature eggs are washed and placed in culture medium inside a temperature- and gas-controlled incubator (about 5–6% CO₂, low oxygen at premier clinics, 37°C).

The semen sample is usually produced on the morning of retrieval, then prepared by density-gradient centrifugation or swim-up to isolate the most motile sperm — the same washing step used in our sperm analysis workup. Fertilisation then happens one of two ways:

  • Conventional IVF — sperm and eggs are cultured together overnight and fertilisation happens naturally in the dish. Used when sperm parameters are normal.
  • ICSI (intracytoplasmic sperm injection) — a single sperm is injected into each mature egg using a micromanipulator. ICSI is preferred for male-factor infertility, prior fertilisation failure, low egg numbers, surgically retrieved sperm (PESA, TESA, TESE), or when genetic testing of embryos is planned.

ICSI typically adds around Rs 40,000 to Rs 1 lakh to the cycle cost in India and is available at all major FOGSI- and ISAR-affiliated clinics. Long-term Australian and European registry data on babies born after ICSI remain reassuring, with low birth-defect rates.

Day 1: The two-pronuclei (2PN) check

What the fertilisation count tells you

If you started with, say, 10 mature eggs and 7 fertilise normally, that is a healthy result — not all eggs are expected to fertilise. The fertilised number is the starting cohort that will narrow over the next few days as the strongest embryos reveal themselves.

Days 2–3: The cleavage stage

Days 2 and 3 are the cleavage stage, when the embryo divides into more and more cells (called blastomeres) without growing much larger. Day-2 embryos usually have 2–4 cells; day-3 embryos usually have 6–8 cells, doubling roughly every 12–18 hours.

Embryologists grade cleavage-stage embryos on cell number, how even the cells are, how much fragmentation is present (tiny pieces of cytoplasm that pinch off during division), and whether any cells have more than one nucleus. A good day-3 embryo typically has 7–9 even cells, under 10–15% fragmentation, and no multinucleation. Slow (fewer than 5 cells), fast (more than 10), very uneven, or heavily fragmented embryos are graded lower. Roughly 60–80% of fertilised eggs reach a good-quality day-3 stage.

Until the early 2000s, day-3 transfer was the standard. Because labs could not reliably pick the best of several day-3 embryos, two or three were often transferred together — which is why older IVF carried a high IVF twins rate. Today, day-3 transfer is reserved for specific situations, such as:

  • Only 1–3 embryos available, where extended culture risks losing them all
  • A history of embryos arresting at day 3–4
  • Lab conditions less suited to blastocyst culture
  • Clear patient preference

Day-3 transfer is still common at some Indian clinics and is entirely appropriate in these scenarios, but the trend at premier FOGSI/ISAR centres is toward day-5 blastocyst transfer when conditions allow.

Day 4: The morula stage and compaction

Day 4 is the morula stage — the embryo compacts into a tight ball of 16–32 cells that looks like a small berry (morula means mulberry in Latin). During compaction the individual cells bond together so tightly that their boundaries blur. This is when the embryo begins its first true specialisation: some cells will become the inner cell mass (the future baby) and others the trophectoderm (the future placenta).

Day-4 embryos are usually not transferred because they are in a transitional state that is harder to grade. Some compact early (day-3 evening), some late (day-4 evening into day 5), and a few never compact properly — a sign of poor developmental potential.

Between day 4 and day 5 the embryo undergoes cavitation: a fluid-filled cavity (the blastocoel) forms inside, pushing the inner cell mass to one side and the trophectoderm to the outer rim. This usually happens 96–110 hours after insemination. Across all stages, roughly 30–60% of fertilised eggs reach the day-5 blastocyst stage — the figure depends on age, ovarian reserve and AMH, egg and sperm quality, and lab conditions. This drop-off is normal biology, not a mistake: it is nature filtering for the embryos most able to implant and continue.

Day 5: The blastocyst stage and Gardner grading

Day 5 brings the blastocyst — the stage that defines modern IVF transfer. A blastocyst has three parts: the blastocoel (fluid cavity), the inner cell mass or ICM (the clump that becomes the baby), and the trophectoderm or TE (the outer layer that becomes the placenta).

Blastocysts are graded on the Gardner scale, used worldwide and by ESHRE, ASRM, ISAR and FOGSI labs. It has three parts:

  • Expansion (1–6): 1 = early blastocyst, small cavity; 2 = cavity over half the embryo; 3 = full blastocyst, cavity fills the embryo; 4 = expanded, thinning shell (zona); 5 = hatching out of the shell; 6 = fully hatched.
  • Inner cell mass (A–C): A = many tightly packed cells (best); B = looser, several cells; C = very few cells.
  • Trophectoderm (A–C): A = many cohesive cells forming a neat layer (best); B = fewer, looser cells; C = very few large cells.

So a 4AA is an expanded blastocyst with top-quality ICM and TE; a 3AB is a full blastocyst with top ICM and good TE; a 3CC is poor on both.

Grade correlates with implantation: top-grade blastocysts (3AA, 3AB, 4AA, 4AB, 5AA, 5AB) implant in about 40–60% of good-prognosis women, while lower grades (CC, BC, CB) sit nearer 10–30%. But grade is a guide, not a guarantee. Lower-grade blastocysts do implant and produce healthy babies, and top-grade ones sometimes fail — because of factors like chromosomal status, uterine receptivity, and plain chance. If you are watching for clues after your transfer, our honest guide to positive signs after embryo transfer explains what actually predicts success and what does not.

Day 6 (and 7): Slow blastocysts and extended culture

Not every embryo reaches the blastocyst stage exactly on day 5. Day-6 blastocysts are still very usable, though their per-embryo implantation rate runs roughly 10–20% lower than a day-5 blastocyst of the same grade. Slower growth may reflect slightly lower developmental potential or simply individual variation.

Day-6 blastocysts are usually frozen by vitrification and used in a later frozen embryo transfer (FET) cycle rather than transferred fresh. Some clinics extend culture to day 7 for very slow embryos; day-7 blastocysts can still be frozen and used, with reported live-birth rates around 10–20% per transfer — valuable when no other embryos are available. ESHRE and ASRM both support case-by-case extended culture to day 7.

Across a full cohort, a rough breakdown is:

  • ~40–60% reach a day-5 blastocyst
  • ~10–20% reach a day-6 blastocyst
  • ~5–10% reach a day-7 blastocyst

The rest arrest earlier. Younger women with good ovarian reserve tend to have more day-5 progression; women over 38 or with diminished reserve tend to have more day-6 to day-7 development. Most Indian clinics freeze day-5 and day-6 blastocysts routinely; day-7 culture is offered at some premier centres. For more on how embryos are stored, see our guide to cryopreservation of sperm, eggs and embryos.

Time-lapse imaging and the EmbryoScope

Time-lapse incubators photograph each embryo every 10–15 minutes throughout the 5–6 day culture, building a video of every cell division, compaction and cavitation — all without ever taking the embryo out of its stable environment. Common systems in India include EmbryoScope, Geri and MIRI.

The practical benefits are real:

  • Fewer incubator door openings, so temperature and gas stay steady
  • Continuous monitoring without disturbing the embryos
  • Timing data (time to first division, to morula, to blastocyst) that can refine selection
  • Detection of abnormal division patterns that predict poorer outcomes

Some systems add scoring software (KIDScore, AI-based iDAScore) that estimates implantation likelihood. However, the evidence that time-lapse actually improves live-birth rates is mixed — a Cochrane review found insufficient proof of benefit over standard incubation, even though embryologists value the extra data for selection.

In India, time-lapse is offered at premier clinics at an extra cost of roughly Rs 25,000 to Rs 60,000 per cycle. ASRM, ESHRE and ISAR consider it promising but not mandatory. Whether to opt in depends on your clinic's protocols, your embryo cohort, and your comfort paying for an optional add-on — the same way you would weigh any other IVF cost and process decision.

PGT-A: genetic testing of embryos before transfer

Preimplantation genetic testing for aneuploidy (PGT-A, formerly PGS) checks whether an embryo has the normal number of chromosomes before it is transferred. It is done at the blastocyst stage by biopsying 5–10 trophectoderm cells — the future placenta, not the future baby — on day 5 or 6. The cells are tested by next-generation sequencing, and embryos are classified as:

  • Euploid — normal chromosome count, suitable for transfer
  • Aneuploid — extra or missing chromosomes, usually not used
  • Mosaic — a mix of normal and abnormal cells, decided case by case

The biopsy is safe in modern labs, with over 95% of embryos surviving biopsy and freezing per ESHRE and ASRM data. The embryo is vitrified while results come back over about 5–14 days, then thawed for a frozen transfer in a later cycle.

PGT-A is generally considered for advanced maternal age (often 38+), recurrent pregnancy loss (two or more miscarriages), repeated implantation failure, severe male-factor infertility, a family history of chromosomal conditions, or by patient request. It can lower miscarriage risk from chromosomal causes, raise the per-transfer success rate in older women, and strongly supports single embryo transfer because one tested euploid blastocyst carries the highest per-embryo chance.

The trade-offs matter too: PGT-A reduces the number of transferable embryos (aneuploid ones are set aside) and may not raise cumulative live-birth rates per stimulation cycle for younger women — only the rate per transfer. In India it typically costs around Rs 25,000 to Rs 40,000 per embryo tested, on top of the cycle. Whether it suits your situation is a decision to make with your reproductive endocrinologist.

Day 3 vs day 5 transfer: how the choice is made

Choosing between a day-3 (cleavage) and a day-5 (blastocyst) transfer is one of the more important calls in an IVF cycle. ESHRE, ASRM and FOGSI/ISAR guidance all favour day-5 blastocyst transfer when enough good embryos exist, because waiting to day 5 lets the lab pick the most developmentally capable embryo, supports single embryo transfer, lowers multiple-pregnancy rates, and gives per-transfer success at least as good as day 3.

Day 5 is usually preferred when:

  • You are under 38 with at least 4–6 fertilised eggs
  • Your day-3 embryos look good, suggesting a capable cohort
  • You prefer transferring fewer embryos

Day 3 may be preferred when:
  • Only 1–3 eggs fertilised (extended culture risks losing them all)
  • Past cycles showed embryos arresting at day 3–4
  • You are over 40 with diminished reserve and every embryo counts
  • Lab conditions favour earlier transfer, or you prefer it

Importantly, the cumulative live-birth rate per stimulation cycle is similar between the two strategies once all transferable embryos are eventually used — the difference is that day 5 gives a higher chance per transfer, while day 3 tends to leave more transferable embryos in the cohort. Many clinics decide based on your actual embryos rather than committing in advance, so the conversation often happens after your day-3 update. If you are still weighing your route to treatment, our IUI vs IVF decision guide sets out when each makes sense.

When to see a doctor (and what to ask)

Embryo development happens entirely in the lab, so there is nothing you can do at home to change it — but there are moments when a focused conversation with your fertility specialist is worthwhile. Raise these clearly:

  • Very low or zero fertilisation — if few or none of your eggs fertilise, ask whether sperm preparation, egg quality, or a switch to ICSI should be reviewed for next time.
  • Repeated developmental arrest — if embryos consistently stop growing around day 3–4 across cycles, ask about egg quality, sperm DNA fragmentation, and whether protocol changes could help.
  • Repeated implantation failure — if good-grade embryos repeatedly fail to implant, ask about uterine evaluation, an endometrial receptivity assessment, and whether PGT-A is appropriate.
  • Recurrent miscarriage after transfer — if you have had two or more losses, you deserve a full workup; our guide to recurrent miscarriage explains the Indian investigation and treatment pathway.

Also seek urgent care for severe abdominal pain, heavy bleeding, breathlessness, or rapid abdominal swelling in the weeks after retrieval — these can signal ovarian hyperstimulation syndrome (OHSS) and need prompt review. While you wait through the transfer and the two-week wait, gentle activity is fine; our guide to safe exercise during IVF covers what to do and avoid.

Myths vs facts

Frequently asked questions

How many of my eggs will become blastocysts?

Roughly 30–60% of fertilised eggs reach the day-5 blastocyst stage, and the journey usually narrows at every step: not all eggs are mature, not all mature eggs fertilise, and not all fertilised eggs make it to blastocyst. Younger women with good ovarian reserve tend to have higher blastocyst rates. A falling number across the days is expected, not a sign something went wrong.

What does an embryo grade like 4AA mean?

On the Gardner scale, the number (1–6) describes how expanded the blastocyst is, the first letter grades the inner cell mass (future baby), and the second grades the trophectoderm (future placenta), with A best and C poorest. So 4AA is an expanded blastocyst with top-quality cells throughout. Grade guides the chance of implantation but never guarantees it — lower-grade embryos still make healthy babies.

Is a day-6 blastocyst worse than a day-5 blastocyst?

A day-6 blastocyst of the same grade implants a little less often than a day-5 one, but it is still very much worth using and frequently leads to healthy pregnancies. Day-6 blastocysts are usually frozen and transferred in a later frozen embryo transfer cycle rather than fresh.

Do I need ICSI, time-lapse, or PGT-A?

None of these is automatically needed. ICSI is mainly for male-factor infertility, prior fertilisation failure, or when PGT-A is planned. Time-lapse and PGT-A help selection in specific situations but are optional paid add-ons. Ask your specialist what each would add in your particular case rather than opting in by default.

Will what I eat or do affect my embryos in the lab?

No — once your eggs are fertilised, the embryos develop entirely in the controlled environment of the lab, and nothing you eat, drink or do can change their growth during these few days. Egg and sperm quality (built over the preceding months) and lab conditions are what matter. Gentle activity, rest and managing stress during the wait are about caring for yourself, not steering the embryos.

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