IVF & ICSI Treatment
A blastocyst is a Day 5 embryo with the highest implantation potential in IVF. Learn how a blastocyst forms day by day, what grades like 4AA and 3AB mean, and when Day 3 transfer is better at Pluro.
The embryos in the IVF process may not be alike, and one of the key landmarks is the development of blastocysts from the embryos. A blastocyst is a relatively advanced embryo, which normally forms on Day 5 following fertilisation. This is unlike the Day 2 or Day 3 embryos which were used in the early days of IVF.
A blastocyst is an embryo at approximately 5 to 6 days following fertilisation. At 5 to 6 days the embryo will contain around 100 to 200 cells, have a fluid filled chamber at its centre, and two distinct cell types, which eventually becomes the fetus and placenta. During IVf the detection of a blastocyst is the most definitive early indicator of development potential.
Three things are worth knowing before the detail. A blastocyst forms on Day 5 or Day 6, not Day 2 or Day 3. Only about 40–60% of fertilised eggs get there. And blastocyst transfer has a higher success rate per transfer, yet a Day 3 transfer is still the better choice for some patients.
Every blastocyst is an embryo, but not every embryo is a blastocyst. “Embryo” covers the whole period from fertilisation onwards. “Blastocyst” is the specific stage an embryo reaches on Day 5 or Day 6, once its cells have organised themselves into two distinct groups around a fluid-filled cavity. What changes in those two days:
In the laboratory, embryologists check the same embryos every day and record what they see. This is what each day looks like in an IVF cycle, once the eggs have been fertilised in the IVF laboratory:
If your blastocyst is a day late that doesn't make it weak automatically! It is common practice for 6 day blastocycts to be frozen and transferred.
A blastocyst has three structures, and they are the reason it can be graded in far more detail than a Day 3 embryo.
What is the significance of blastocyst formation in this context? The reason is that the embryos with strong developmental capacity are the ones which develop to blastocyst stage. Embryo culture to Day 5 naturally selects those embryos which would have failed to establish a pregnancy. In simple terms, this implies that blastocyst formation increases the implantation potential of the embryos.
Blastocyst culture also gives the lab a clearer choice. If 2 or 3 embryos survive until day 5, they are all at a quality that they may be used by the embryologist and one chosen to be transferred and the other(s) are frozen to await either frozen embryo transfer, or for an embryo banking to be built. However, to state quite boldly, a perfect blastocyst is not a certainty and although good-quality embryos can still fail to implant the implantation factors are still as critical as the quality of the embryo.
Most laboratories, including ours, grade a blastocyst on three things at once, and the grade is read in that order — expansion, then inner cell mass, then trophectoderm.
So 4AA is an expanded blastocyst with top-quality cells in both groups, and 3AB is a full blastocyst with an excellent inner cell mass and a slightly less even placental layer.
Grades describe appearance on one particular day, not chromosomes. A 3BB blastocyst can be genetically normal and a 4AA can be abnormal, which is why grading and genetic testing answer two different questions.
At Dr. Modi IVF Center in Nagpur, Dr. Modi and the embryology team monitor embryo development carefully. The choice of whether to recommend a day 3 or a day 5 transfer also depends on your own unique circumstances: number of developing embryos, their quality on day 3, history of previous treatments. A greater number of embryos doesn't necessarily mean day 5 is beneficial in all circumstances.
Blastocyst culturing also simplifies performing PGT (preimplantation genetic testing) if it needs to be done ( e.g in women with history of recurrent miscarriage, or family history of genetic diseases). The embryo can be sampled and tested before the embryo transfer, giving increased likelihood of a normal pregnancy.
Blastocyst transfer gives a higher chance of pregnancy per transfer than a Day 3 transfer, but the number that matters to you is your own. Four things move it most:
Cycle averages can often be deceptive; it's worth realizing what IVF success rates really mean when comparing between clinics. Request the success rate for your age category and stage of embryo development, rather than the stated "headline" number from your team.
If you've undergone Day 3 transfers in a prior IVF and are contemplating whether blastocyst culture may yield better results-speak to Dr. Modi about this when you meet with her.
Q: Is blastocyst transfer always better than Day 3 transfer?
A: Not necessarily. Success rates per transfer of blastocysts are usually higher, but you will only be culturing Day 5 embryos at the risk of them possibly not surviving at all for transfer (if your supply is very low). This will be decided with your embryologist and by Dr. Modi based on your particular cycle.
Q: Can all embryos reach the blastocyst stage?
A: No. Usually 40-60% of fertilized eggs would reach blastocyst stage, which is also subject to the quality of eggs and sperms. In some cycle, few fertilitized egg resulted into 1 to 2 blastocysts, that also is very common.
Q: What does the blastocyst grading mean?
A: Blastocysts are graded on expansion and also on the quality of the inner cell mass (ICM - that which turns into the baby) and the trophectoderm (TE - that which turns into the placenta). Grades like 4AA or 3AB show ICM/TE quality and expansion. Your embryo grades will be clearly explained by Dr. Modi's team.
Q: What does “blast” mean in an IVF report?
A: “Blast” is laboratory shorthand for blastocyst, a Day 5 or Day 6 embryo. If your report says “2 blasts frozen”, two embryos reached the blastocyst stage and were vitrified for later use. A code beside it, such as 4AA, is the grade for expansion and cell quality.
Q: What does blastocyst mean in pregnancy?
A: Blastocyst stage begins with the onset of pregnancy. In spontaneous cycles pregnancy begins, the blastocyst appears about 5-6 days postfertilisation then moves to the uterus and implants into the lining around day 6-10 postfertilisation. Only after implantation does it develop into what is called an embryo proper, and a beta hCG blood test is the first reliable confirmation.
Not always. Blastocyst transfer has higher per-transfer success rates, but if a patient has very few embryos, culturing to Day 5 risks having no embryos survive to transfer. Your embryologist and Dr. Modi will decide together based on your specific cycle.
No. Typically 40–60% of fertilised eggs reach blastocyst, depending on egg and sperm quality. Some cycles produce only 1–2 blastocysts from several fertilised eggs, this is normal and reflects natural selection.
BLASTOCYSTS are evaluated based on their expansion (1–6 scale) and the quality of two cell groups: the inner cell mass (ICM, which becomes the baby) and the trophectoderm (TE, which becomes the placenta)। Grades like 4AA or 3AB show ICM/TE quality and expansion.
