One of the most anxious periods in an IVF cycle is the time between egg retrieval and embryo transfer. Specifically, the stretch between day 3 and day 5 of embryo development is when much of the critical biology happens, and when patients often hear updates that raise as many questions as they answer.
Understanding what is actually occurring inside the incubator during these days, what blastulation is, why it matters, and what survival rates typically look like, helps patients make sense of the numbers they receive and engage more meaningfully in decisions about their own care.
At Hanabusa IVF, the embryology team communicates embryo development updates clearly, so patients understand not just what is happening but what it means for their cycle.
Embryo Development: A Brief Timeline
After fertilization, embryo development follows a predictable sequence. In the first two days, the fertilized egg, now called a zygote, undergoes its first cell divisions. By day 3, a healthy embryo has typically reached the 8-cell stage, sometimes called a cleavage-stage embryo. By day 5, embryos that continue to develop reach the blastocyst stage, a more complex structure with differentiated cell populations.
In clinical IVF practice, most embryos are cultured through to day 5 or day 6 before transfer or freezing, rather than being transferred at day 3 as was more common in earlier IVF. This shift reflects the significant evidence that blastocyst-stage transfer allows for better embryo selection and is associated with higher implantation rates per transfer.
What Is Blastulation?
Blastulation is the process by which a day 3 cleavage-stage embryo transitions into a blastocyst. During this transition, the embryo undergoes compaction and then begins to form a fluid-filled cavity called the blastocoel. The cells differentiate into two distinct populations: the inner cell mass (ICM), which will become the fetus, and the trophectoderm (TE), which will become the placenta.
An embryo that successfully reaches the full blastocyst stage by day 5 has demonstrated the metabolic and developmental capacity to organize complex cell structures. This is a meaningful developmental milestone, and one that many embryos do not reach.
"Blastulation is not just a technical milestone. It reflects real biological competence. An embryo that reaches a well-expanded blastocyst has shown us something important about its ability to organize and function, and that has clinical relevance for what we expect from transfer."
— Dr. Lyndon Chang, Medical Director at Hanabusa IVF
What Happens to Day 3 Embryos That Do Not Reach Day 5?
Not all day 3 embryos successfully reach the blastocyst stage. Some embryos arrest between day 3 and day 5, stopping development before forming a blastocyst. Arrest at this stage typically reflects chromosomal abnormalities in the embryo, though deficiencies in cytoplasmic quality or other factors can also contribute.
Embryo arrest does not represent a failure of the IVF process. It reveals information about the embryos that was always true but not previously visible. The arrest happens because the embryo lacks the biological capacity to continue development, and identifying this before transfer is preferable to a failed implantation.
The proportion of day 3 embryos that successfully reach blastocyst varies significantly by patient age and ovarian reserve. For younger patients with good egg quality, a majority of fertilized eggs may reach blastocyst. For older patients or those with egg quality challenges, a smaller proportion will make this transition, which is one of the reasons Hanabusa IVF's individualized approach to stimulation and embryo culture is particularly relevant for complex cases.
Why Day 5 Blastocysts Are Preferred for Transfer
Extending culture to day 5 allows the embryology team to observe which embryos demonstrate the developmental capacity to form a blastocyst. This self-selection process means that the embryos available for transfer have already passed through one significant developmental checkpoint. Blastocyst-stage embryos also are at the right developmental stage for uterine synchrony at the time of a day 5 frozen embryo transfer, which aligns embryo development with the natural window of implantation.
At Hanabusa IVF, blastocyst-stage transfer and cryopreservation are standard practice. Embryos not used in an immediate transfer cycle are vitrified at the blastocyst stage for future use.
Day 6 Blastocysts: Are They as Good as Day 5?
Some embryos reach the blastocyst stage on day 6 rather than day 5. Day 6 blastocysts are generally considered slightly lower prognosis than day 5 blastocysts of the same grade, as the delay suggests slower development. However, day 6 blastocysts are not unusable. Many have been successfully transferred and resulted in healthy pregnancies. The decision about whether and when to transfer a day 6 blastocyst is made based on the full picture of the patient's embryo cohort, testing results, and individual circumstances.
"I get questions all the time about whether a day 6 blastocyst is a write-off. The honest answer is no. A day 6 blastocyst is a slower developer, not necessarily a non-viable one. We look at the complete picture before making transfer recommendations."
— Dr. Diana LeBlanc, a Fertility Specialist at Hanabusa IVF
What Good Embryo Culture Requires
Embryo survival from day 3 to day 5 is not only about the embryo itself. The laboratory environment matters significantly. Temperature stability, gas composition, culture media quality, and handling practices all influence how embryos develop in the incubator. Hanabusa IVF's laboratory holds CAP and ABB accreditation and uses CryoSentinel and XiltriX monitoring systems to maintain real-time oversight of laboratory conditions. The team's more than a decade of experience in vitrification and blastocyst culture means that the support the embryo receives in the lab is optimized for its development.
Frequently Asked Questions
How many of my embryos will make it to day 5?
This depends significantly on your age and egg quality. For younger patients under 35 with good ovarian reserve, a majority of fertilized eggs may reach blastocyst. For patients over 38 or those with egg quality challenges, the proportion is often lower. Your physician at Hanabusa IVF will give you context for what to expect based on your specific situation.
Is it bad if an embryo only reaches day 3 and does not make it to day 5?
An embryo that arrests between day 3 and day 5 has revealed important biological information. While it is not the outcome anyone hopes for, arrest does not indicate a problem with the IVF process itself. It most commonly reflects chromosomal or developmental limitations in the embryo.
Why do some clinics transfer embryos on day 3?
Day 3 transfer was standard practice in earlier IVF and remains appropriate in some specific situations, such as when a patient has very few embryos and the clinical team prefers not to risk all of them arresting in the incubator. For most patients today, however, extended culture to day 5 is the preferred approach because it allows for better embryo selection.
Can a low-grade day 5 blastocyst still result in pregnancy?
Yes. Embryo grade reflects morphology, not chromosomal status. A lower-grade blastocyst can be chromosomally normal and result in a successful pregnancy. Chromosomal testing (PGT-A) provides information about chromosomal status that morphology grading alone cannot reveal.
What happens to embryos that do not survive to day 5?
Embryos that arrest before reaching blastocyst are not used for transfer or freezing. They are noted in your embryo report. Understanding how many embryos developed to blastocyst out of how many fertilized is one of the key data points your care team uses to counsel you on next steps.
What the Numbers Mean for Your Specific Cycle
Embryo development from day 3 to day 5 is one of the most information-rich phases of an IVF cycle. The number of embryos that reach blastocyst, the quality of those blastocysts, and how they develop across that window all inform the clinical picture. At Hanabusa IVF, patients receive clear communication about each stage of embryo development and what the findings mean for their options going forward.
If you have questions about embryo culture, blastocyst development, or what to expect from your own IVF cycle, a consultation at Hanabusa IVF is the right starting point.
