Embryo development day by day follows a predictable path. On day 1, the egg is checked for signs of fertilisation; on days 2 and 3, it divides into about 4 and then 8 cells; on day 4, the cells pack tightly into a ball called a morula; and on day 5 or 6, it becomes a blastocyst, a hollow ball of around a hundred or more cells that is ready to hatch and implant. Not every embryo completes this journey, and that is a normal part of how nature selects the strongest ones. At Yaal Fertility Centre in Chennai, Dr. Rukkayal Fathima and our team explain each stage to couples during IVF, so the daily updates from the lab make sense.
What Is an Embryo?
An embryo is the earliest stage of human development, starting when a sperm fertilises an egg. In everyday medical use, it is called an embryo from fertilisation until the end of the eighth week after conception, which is about ten weeks into a pregnancy counted from the last period. After that, it is called a fetus.
Embryo Meaning in Simple Words
Think of the embryo as the very first version of a baby: a single cell carrying half its genetic instructions from the mother and half from the father. Over the first week it divides again and again, organises itself into two kinds of cells, and prepares to attach to the lining of the womb.
Zygote, Embryo and Fetus
| Term | When | What it is |
|---|---|---|
| Zygote | Day 1 | The single fertilised cell, before it divides |
| Cleavage-stage embryo | Days 2 to 3 | An embryo of roughly 2 to 8 cells |
| Morula | Day 4 | A compact ball of cells |
| Blastocyst | Days 5 to 6 | A hollow ball with an inner cell group and an outer layer |
| Embryo (in pregnancy) | Until about 10 weeks of pregnancy | Body structures and organs begin to form |
| Fetus | From about 10 weeks of pregnancy | The baby grows and matures |
In IVF, people usually say "embryo" for every stage from day 1 onwards, and that is how we use the word in this guide.
Embryo Development Day by Day in the IVF Lab
In IVF, the first days of development happen in the laboratory, where embryologists can watch them. Our guide to what IVF is explains the whole treatment; here we focus only on what happens to the embryo.
Day 0 to Day 6 at a Glance
| Day | Stage | What happens | What the embryologist checks |
|---|---|---|---|
| Day 0 | Egg collection and fertilisation | Eggs are collected and mixed with sperm (IVF) or injected with one sperm (ICSI) | How many eggs are mature |
| Day 1 | Zygote | Signs of fertilisation appear | Whether each egg fertilised normally |
| Day 2 | Early cleavage | The cell divides into about 2 to 4 cells | Number and evenness of cells |
| Day 3 | Cleavage stage | About 6 to 8 cells | Cell number, evenness, small fragments |
| Day 4 | Morula | Cells pack tightly into a ball | Usually left undisturbed |
| Day 5 | Blastocyst | A fluid-filled cavity forms; two cell types appear | Expansion, inner cell group and outer layer |
| Day 6 | Expanded or hatching blastocyst | The blastocyst grows and starts to break out of its shell | Same grading as day 5 |
These are typical timings. Some healthy embryos are a little slower or faster, which is why the lab looks at the overall pattern rather than one moment.
Day 0: Egg Collection and Fertilisation
On the day of egg collection, the eggs are checked under a microscope. Only mature eggs can be fertilised. In conventional IVF, each egg is placed with many prepared sperm and fertilisation happens on its own. In ICSI, an embryologist injects a single sperm directly into each mature egg, which is often used when sperm count or movement is low. Our ICSI vs IVF comparison explains when each is chosen. The IVF process timeline covers what happens to you before and after this day.
Day 1: The Fertilisation Check
The morning after, the embryologist looks for two small round structures inside the egg called pronuclei: one carries the mother's genetic material and one carries the father's. This is often written as 2PN, and it means normal fertilisation.
Sometimes an egg shows one pronucleus, or three or more. These are signs of abnormal fertilisation, for example when two sperm enter one egg. Such embryos are usually not used, because they rarely develop normally. Some eggs do not fertilise at all. It is common for not every mature egg to fertilise, and your doctor will tell you how many did.
Days 2 and 3: Cleavage
The fertilised egg now divides without growing in overall size, so each new cell is smaller. By day 2 there are usually 2 to 4 cells, and by day 3 about 6 to 8. This is called the cleavage stage.
Around day 3, something important happens: the embryo starts using its own genes. Until then it has run on supplies stored in the egg. If there is a problem with the embryo's chromosomes, this is often the point where it stops growing.
Some clinics transfer embryos on day 3, while many prefer to wait until day 5. We compare the two in our guide to day 3 vs day 5 blastocyst transfer, so we will not repeat it here.
Day 4: Morula
On day 4, the cells press tightly together so that their edges are hard to see. The embryo now looks like a small, smooth ball and is called a morula, from the Latin word for mulberry. This packing, called compaction, prepares the cells to take on different jobs. Many labs do not check embryos on day 4 and give no update that day, so a quiet day 4 is normal.
Day 5: Blastocyst
By day 5, fluid collects inside the embryo and forms a cavity. The cells have now split into two groups:
- Inner cell mass: a small cluster on one side that will become the baby.
- Trophectoderm: the outer layer of cells that will become the placenta.
An embryo at this stage is called a blastocyst. It usually has around a hundred or more cells, though the exact number varies. Reaching the blastocyst stage is a good sign that the embryo has passed several natural checkpoints.
Day 6: Expansion and Hatching
As the blastocyst fills with more fluid, it expands and its outer shell, called the zona pellucida, becomes thin. The blastocyst then starts to squeeze out through a small gap. This is called hatching, and it must happen before the embryo can attach to the womb lining.
Some embryos reach the blastocyst stage on day 6 instead of day 5. Day 6 blastocysts can still lead to healthy pregnancies, and they are often frozen for transfer in a later cycle.
Some clinics use a technique called assisted hatching, which makes a small opening in the shell, in selected situations. It is not needed for everyone, and your doctor will discuss it only if it applies to you.
How Embryos Are Graded
Embryo grading is a way of describing how an embryo looks under the microscope. It helps the team decide which embryo to transfer first and which to freeze. Grading systems differ slightly between labs, but the ideas are the same.
Day 3 Grading
On day 3, embryologists look at three things:
- Number of cells: about 6 to 8 is typical.
- Evenness: whether the cells are similar in size.
- Fragments: small pieces of cell material that break off. A few fragments are common; a lot can lower the chance of further growth.
Labs record this as a number or a short code, such as "8-cell, grade 1". The best way to understand your own report is to ask your doctor what each part means in your lab's system.
Blastocyst Grading: What "4AA" Means
Blastocysts are commonly graded with a number and two letters, for example 4AA, 3BB or 5AB.
| Part of the grade | What it describes | How to read it |
|---|---|---|
| Number (1 to 6) | How expanded the blastocyst is | 1 = early cavity; 3 = cavity fills the embryo; 4 = expanded; 5 = hatching; 6 = fully hatched |
| First letter (A, B or C) | The inner cell mass, which becomes the baby | A = many tightly packed cells; B = fewer, looser cells; C = very few cells |
| Second letter (A, B or C) | The outer layer, which becomes the placenta | A = many cells in a smooth layer; B = fewer cells; C = very few, uneven cells |
So 4AA describes an expanded blastocyst with a strong inner cell group and a strong outer layer. 3BB describes a full blastocyst with good, slightly looser cell groups. Both are commonly transferred or frozen.
Why the Grade Is Not Everything
It is easy to worry about letters on a report, but grading has clear limits:
- It describes appearance only. It cannot show whether the embryo has the right number of chromosomes. A top-grade embryo can have a chromosome problem, and a lower-grade embryo can be completely normal.
- Lower grades still lead to healthy babies. Many children are born from embryos graded B or even C in one category.
- It is a snapshot. Embryos change hour by hour, and a grade reflects the moment it was checked.
- Other factors matter too. The woman's age, the womb lining and overall health all affect whether an embryo implants.
If chromosome information is needed, for example after repeated miscarriage, some couples choose genetic testing of embryos. Our complete guide to PGT explains how it works and who it may help.
Why Do Some Embryos Stop Growing?
One of the hardest parts of IVF is watching the number fall: so many eggs collected, fewer fertilised, fewer still on day 3, and only some reaching blastocyst. This drop is expected, and it is the same thing that happens unseen in natural conception every month.
Normal Attrition at Each Stage
At every step, some embryos stop developing. Commonly, fewer than half of fertilised eggs reach the blastocyst stage, though this varies widely from couple to couple and cycle to cycle. Your doctor will explain what is typical for your age and situation, rather than a single number that applies to everyone.
Common Reasons for Embryo Arrest
- Chromosome problems: the most common reason. An embryo with missing or extra chromosomes often cannot organise itself into a blastocyst.
- Egg quality and age: as women get older, a larger share of eggs have chromosome errors, and eggs have less energy to fuel division.
- Sperm factors: damage to the DNA in sperm may not stop fertilisation but can affect growth later, often after day 3.
- Chance: sometimes an embryo stops for no reason that can be found.
What It Means for You
An embryo stopping is not caused by anything you did, ate or felt during the cycle. If very few embryos reach blastocyst in more than one cycle, your doctor may look more closely at egg quality, sperm DNA, or the stimulation and lab plan, and discuss changes for next time.
Embryo Freezing and Thawing
Good-quality embryos that are not transferred straight away can be frozen for later use.
How Embryos Are Frozen
Today, embryos are frozen with a very fast method called vitrification. The embryo is placed in protective solutions that draw out water, then cooled so quickly in liquid nitrogen that ice crystals do not have time to form. This protects the delicate cells. Embryos are most often frozen at the blastocyst stage.
Thawing and Transfer
When you are ready, the embryo is warmed, rinsed out of the protective solutions and checked. Most good-quality blastocysts survive warming well. The embryo is then placed in the womb in a carefully timed cycle. Our guide to frozen embryo transfer explains how that cycle is planned.
If you are deciding whether to freeze eggs or embryos, our comparison of embryo freezing vs egg freezing sets out the differences, and the embryo freezing page explains the treatment.
Embryo Development in Natural Conception vs IVF
The stages are exactly the same in a natural pregnancy. The difference is where they happen.
The Natural Journey
In natural conception, the egg is fertilised in the fallopian tube, usually within a day of ovulation. The embryo then divides as it is slowly moved along the tube, reaches the womb around day 4 or 5, becomes a blastocyst, hatches and starts to implant into the lining around 6 to 10 days after ovulation.
Side by Side
| Day | Natural conception | IVF |
|---|---|---|
| Day 0 | Ovulation; egg meets sperm in the tube | Egg collection; IVF or ICSI in the lab |
| Day 1 | Fertilised egg in the fallopian tube | Fertilisation check under the microscope |
| Days 2 to 3 | Dividing embryo travels along the tube | Cleavage-stage embryo in the lab |
| Day 4 | Morula reaches or nears the womb | Morula, usually left undisturbed |
| Days 5 to 6 | Blastocyst floats in the womb, then hatches | Blastocyst graded, then transferred or frozen |
| Days 6 to 10 | Implantation into the womb lining | After a day 5 transfer, implantation usually begins within a day or two |
| What you can see | Nothing until a pregnancy test turns positive | Daily lab updates on each embryo |
After the Embryo Implants
Once the blastocyst attaches, its outer layer begins making the pregnancy hormone hCG. A blood test about 9 to 10 days after a day 5 blastocyst transfer measures this hormone; see our guide to the pregnancy test after IVF. Some women notice mild spotting or cramps around implantation, while many notice nothing at all. Our article on implantation symptoms explains what is normal.
Questions to Ask About Your Embryo Report
- How many eggs were mature, and how many fertilised normally?
- How many embryos are still growing today?
- What do the grades on my report mean in your lab's system?
- Are we planning a day 3 or day 5 transfer, and why?
- How many embryos might be frozen?
- If few embryos reached blastocyst, what could we change next time?
Writing these down before your updates helps you understand each call from the lab.
Myths About Embryo Development
| Myth | Fact |
|---|---|
| Every fertilised egg should become a blastocyst | Many stop growing at each stage; this is normal in IVF and in nature |
| Only "AA" embryos lead to pregnancy | Many healthy babies are born from embryos with B or C grades |
| A top grade means the embryo is genetically normal | Grading shows appearance only, not chromosomes |
| Day 6 blastocysts are poor quality | Day 6 blastocysts can still lead to healthy pregnancies |
| Frozen embryos are weaker than fresh ones | Modern freezing preserves embryos well; frozen transfers are routine |
| Embryos stop growing because of stress or diet during the cycle | Arrest is mostly due to chromosomes and egg or sperm quality |
Frequently Asked Questions About Embryo Development
How many cells should an embryo have on day 3?
Usually about 6 to 8 cells. Embryos with slightly fewer cells can still go on to become healthy blastocysts.
What is a blastocyst?
A blastocyst is an embryo on day 5 or 6 with a fluid-filled cavity, an inner group of cells that will become the baby and an outer layer that will become the placenta.
What does 4AA mean in embryo grading?
The 4 means the blastocyst is expanded, the first A means the inner cell group is strong, and the second A means the outer layer is strong. It describes appearance, not genetic health.
Can a low-grade embryo result in a healthy baby?
Yes. Lower-grade embryos regularly lead to healthy pregnancies, because grading cannot see chromosomes and is only one of many factors in implantation.
Why did my embryos stop growing after day 3?
The most common reason is a chromosome problem, which often shows up around day 3 when the embryo starts using its own genes. Egg and sperm quality also play a part.
Is a day 6 blastocyst as good as a day 5?
Day 6 blastocysts are a little slower but can still lead to healthy pregnancies. They are often frozen and transferred in a later cycle.
When does an embryo implant after transfer?
After a day 5 blastocyst transfer, the embryo usually hatches and begins to implant within a day or two, and a blood pregnancy test is usually done 9 to 10 days after transfer.
IVF and Embryo Care at Yaal Fertility Centre, Chennai
At Yaal Fertility Centre, Dr. Rukkayal Fathima, MRCOG (UK) and our team keep you informed at every stage of your IVF treatment, from the fertilisation check to the decision about which embryo to transfer and which to freeze. We explain your embryo report in plain words, including what the grades do and do not mean, so you can make each decision with confidence.
You can meet our team at any of our eight centres across Chennai: Kilpauk, Mylapore, Thiruvanmiyur, Pallikaranai, Madipakkam, Red Hills, Velappanchavadi and Tambaram.
If you are planning IVF or would like a second opinion on an embryo report, book a consultation with Yaal Fertility Centre.
Written by
Dr. Rukkayal Fathima
Specialist at Yaal Fertility Centre, Chennai. Providing expert guidance on fertility and women's health.
