Blastocyst embryo: what your IVF lab updates mean

Expect the numbers in your lab updates to get smaller as the week goes on. Not every egg collected is mature, not every mature egg fertilizes, and not every embryo keeps growing. Cleveland Clinic says that, on average, about 70% of mature eggs fertilize and about half of fertilized eggs reach the blastocyst stage. The sections below explain the words the lab uses.

The lab checks your embryos and contacts you on its own reporting schedule, which depends on your treatment plan, so you may not hear something every day. Your body is also still healing from retrieval, so the warning signs are at the end of this page.

How the lab counts the days

The day of your egg retrieval is Day 0, and each update is counted from it.

On Day 0, you will hear how many eggs were collected. This total counts both mature and immature eggs. A mature egg is one that is ready to be fertilized. Some eggs may be damaged at retrieval. They may be in the count, but they can't be fertilized or frozen.

Day 1: the fertilization check

In the lab, the eggs are joined with sperm in one of two ways:

• Standard IVF: many swimming sperm are put next to each egg in a dish, and one sperm gets in on its own.
• ICSI: an embryologist puts one sperm right into the egg (more on this below).

The next day, the lab looks for signs of fertilization. It usually looks for two pronuclei, one from the egg and one from the sperm, as a sign of normal fertilization. Your report may call this 2PN or a zygote. Other findings, such as one, three or no visible pronuclei, need interpretation by the embryology team. A single check can miss pronuclei that have already faded, so an egg with none seen can still develop normally. Some eggs with only one have led to healthy births, and international lab guidance supports using them only with caution. It does not recommend using eggs with three. Ask your clinic how its lab handles these findings.

Not every mature egg will fertilize. In rare cases, even ICSI does not lead to fertilization.

Days 2 and 3: the cleavage stage

Now the embryo starts to divide. Labs call this the cleavage stage. A day 3 embryo has about 6 to 12 cells. On Day 3, the lab may score each embryo under a microscope, looking at:

• Cell number: how many times the cells have split.
• Symmetry: whether the cells are about the same size.
• Fragmentation: small bits of cell debris. Less is better.

Days 5, 6 and 7: the blastocyst stage

A blastocyst is defined by how far the embryo has developed, not by its age. It is a ball of cells with three main parts:

• The inner cell mass, a cluster of cells that can grow into the baby.
• The trophectoderm, the outer layer of cells. It becomes the placenta.
• A cavity filled with fluid.

Developing embryos should reach this stage by day 5 or 6, and rarely day 7. Reaching day 5 does not make an embryo a blastocyst, and not every embryo gets there.

Is a day 6 blastocyst a problem?

Embryos grow at different speeds. A day 6 blastocyst has taken longer to reach this stage, and day 6 and even day 7 blastocysts have led to live births. Slow-growing blastocysts may have a lower chance overall, though.

Earlier studies suggested the gap between day 5 and day 6 went away when embryos had normal chromosome test results. A 2024 study from one hospital in China looked back at 775 first frozen transfers of a single embryo with normal results. The patients had testing because of a known genetic condition or chromosome change. Day 5 blastocysts led to more live births than day 6. The authors say day 6 blastocysts can still give reasonably good results in some situations, and better trials are needed. Ask your doctor how your clinic uses day 6 and day 7 embryos.

What does an embryo grade mean?

A grade is a short code that describes how an embryo looks. It is not a test of its genes.

Many labs use a blastocyst system with one number and two letters, like "4AB":

Part of the grade	What it describes
The number (1 to 6)	How far the blastocyst has expanded
The first letter (A, B or C)	The inner cell mass, which can become the baby
The second letter (A, B or C)	The outer layer, which becomes the placenta

Not every lab uses this system. Some use fewer levels, or words like good, fair and poor. Ask your clinic to explain its own scale.

Here is what grades can and can't tell you:

• Grading is one tool your doctor uses, along with your own health details.
• Two embryologists may grade the same embryo differently.
• Researchers still debate how well grades can rank one blastocyst against another.
• Embryos with a "C" have still led to live births.

What is ICSI?

ICSI stands for intracytoplasmic sperm injection. With a tiny needle, the embryologist puts a single sperm directly into the egg. Your clinic may suggest ICSI if:

• The sperm count, movement or shape is low.
• Eggs did not fertilize in a past IVF cycle.
• You plan some kinds of genetic testing on embryos.
• You are using frozen eggs or sperm taken out by surgery.

Many labs also use ICSI as a routine step. ASRM's ICSI fact sheet says ICSI fertilizes 50% to 80% of eggs. Some eggs may be damaged during the process, and some embryos stop growing.

What is PGT-A testing?

PGT-A stands for preimplantation genetic testing for aneuploidy. "Aneuploidy" means an embryo has the wrong number of chromosomes, which is closely linked to miscarriage.

If you choose PGT-A, the lab removes a few cells from each embryo being tested and checks them for the wrong number of chromosomes. Results can come back as normal (euploid), abnormal (aneuploid), mosaic (the sample shows a mix of cells with different chromosome counts), or with no clear result. Ask about each of these before testing, including what your clinic does with mosaic embryos.

Keep these points in mind:

• PGT-A is a screening test. It can't promise a pregnancy or a healthy baby.
• ASRM says the value of PGT-A as a routine test for everyone doing IVF has not been shown.
• Whether it fits you depends on your age, history and wishes.
• Freezing the embryos gives time for the results to come back. Read more about freezing embryos and freeze-all cycles.

Some people may be offered other tests. PGT-M looks for a known inherited condition. PGT-SR looks for a known change in chromosome structure.

What happens to each embryo

What happens to each embryo depends on what the lab finds, the consent forms you signed and your clinic's policies. Some embryos may be transferred or frozen. Embryos that stop developing are not used, and clinics are advised to have a written policy on embryos with abnormal or mosaic results, shared with you before testing. Ask your clinic to explain its plan before decisions are made.

Questions to ask your clinic

• When will you send updates, and how? By phone or patient portal?
• What grading scale does your lab use?
• Will you use ICSI, standard IVF or both? Why?
• Do you freeze or test day 6 and day 7 blastocysts?
• Is PGT-A a good fit for us, and what does it cost?
• Which embryos are suitable for freezing or transfer, and what is the plan for each?

When to get help

Call your clinic right away, day or night, if you have:

• A fever of 100.4°F (38°C) or higher.
• Heavy bleeding: soaking a full-size pad with bright red blood in less than an hour, or bleeding heavier than a period.
• Belly or pelvic pain that is sudden or severe, gets worse, or isn't eased by the pain relief your clinic recommended.
• Pain and bloating in your belly, or bloating that is getting worse.
• Nausea and vomiting, nausea that won't go away, or you can't keep fluids down.
• Peeing less than usual, even if you are not thirsty.
• Fast weight gain. Your clinic will tell you its number, but do not wait to reach a weight threshold if you feel unwell.
• Dizziness, lightheadedness, or feeling faint.
• Pain, redness or warmth in an arm or leg.
• Blood in your urine, or pain when you pee or poop.
• Discharge from your vagina that smells bad.

Tell your clinic at your next contact about other milder symptoms that are new, don't go away or slowly get worse.