From Fertilization to Blastocyst: What Happens to Your Embryo on Days 1 Through 6

The IVF lab as an incubator

After egg retrieval, fertilized eggs are placed in specialized culture media inside temperature-controlled, gas-regulated incubators designed to mimic conditions inside the fallopian tube. The lab maintains strict pH, temperature, oxygen concentration, and humidity. Embryos develop in microdroplets of medium under oil to prevent evaporation.

Modern labs use either traditional incubators where dishes are removed briefly for daily assessment, or time-lapse incubators that photograph the embryos at intervals without disturbing them. Time-lapse imaging provides more data points for the embryologist but has not been shown to improve live birth rates compared to standard observation in large trials.

The lab environment matters enormously. Volatile organic compounds in the air, temperature fluctuations during dish handling, and media quality all affect embryo development. A good embryology lab is as much about engineering and quality control as it is about biology.

Day 0: egg retrieval and insemination

On the day of retrieval, the embryologist identifies mature eggs (metaphase II) from the follicular fluid collected during the procedure. Mature eggs have extruded the first polar body and are ready for fertilization. Immature eggs (metaphase I or germinal vesicle stage) may be cultured for a few hours to see if they mature, but their fertilization and development rates are generally lower.

Insemination happens three to six hours after retrieval, either by placing prepared sperm around each egg in a dish (conventional IVF) or by injecting a single sperm directly into each mature egg (ICSI). The choice depends on sperm quality, prior fertilization history, whether PGT is planned, and clinic preference.

Day 1: fertilization check

Approximately 16 to 18 hours after insemination, the embryologist checks each egg for pronuclei, the small circular structures visible under the microscope that represent the maternal and paternal genetic contributions preparing to merge.

Normal fertilization shows two pronuclei (2PN). This is the result the lab is looking for. One pronucleus (1PN) may indicate parthenogenetic activation or an observation timing issue. Three or more pronuclei (3PN) indicate polyspermic fertilization, meaning more than one sperm entered the egg, or a failure of the egg to extrude the second polar body. Abnormally fertilized eggs are set aside because they cannot produce viable embryos.

The fertilization rate, meaning the percentage of mature eggs that show 2PN, is typically 60 to 80 percent with conventional IVF and 70 to 85 percent with ICSI. Rates vary based on egg maturity, sperm quality, and lab conditions. Total fertilization failure, where no eggs fertilize, occurs in roughly 5 to 10 percent of conventional IVF cycles and less than 3 percent of ICSI cycles.

Day 2: the first cell divisions

By day two, normally developing embryos have divided into two to four cells. Each cell is called a blastomere. The embryologist assesses cell number, cell symmetry, and the presence of fragmentation, which are small membrane-bound cellular fragments that are not full cells.

Mild fragmentation (less than 10 percent of the embryo volume) is common and generally does not affect development. Moderate fragmentation (10 to 25 percent) is associated with somewhat lower implantation rates. Severe fragmentation (above 25 percent) is a negative prognostic sign.

At this stage the embryo is running almost entirely on energy stores from the egg. The embryonic genome has not yet activated. This means that egg quality is the dominant factor in early development, which is why age, which affects egg quality, is such a strong predictor of IVF outcomes.

Day 3: the eight-cell stage and genomic activation

By day three, a normally developing embryo has six to ten cells, with eight cells being the textbook number. Between the four-cell and eight-cell stage, the embryonic genome activates. This is a critical transition: the embryo shifts from relying on maternal RNA and proteins stored in the egg to using its own newly activated genes.

Embryos that arrest at the four-to-eight-cell stage often do so because of genomic activation failure. The embryo's own DNA may carry errors, such as chromosomal aneuploidies, that prevent it from successfully taking over cellular control from the maternal machinery.

Historically, many clinics transferred embryos on day three. Day-three transfer is still done in some situations, but the trend has shifted strongly toward day-five transfer because extending culture to blastocyst provides additional selection. Embryos that reach blastocyst have demonstrated the ability to activate their genome and continue dividing, which is a meaningful quality filter.

The day-3-to-blastocyst attrition

It is normal for a significant percentage of day-three embryos to arrest before reaching blastocyst. A commonly cited benchmark is that roughly 40 to 60 percent of normally fertilized eggs reach blastocyst, though this varies widely based on patient age, lab quality, and the specific culture system used. The drop-off is not a lab failure. It reflects the biological reality that not all embryos are capable of sustained development.

Day 4: compaction and morula formation

On day four, the individual blastomeres begin to compact, pressing tightly together and losing their distinct cell boundaries. The embryo is now called a morula. Compaction is driven by the formation of tight junctions and gap junctions between cells, which allow them to communicate and coordinate.

The morula stage is a brief transitional phase. Most labs do not formally assess embryos on day four because the compacted appearance makes it difficult to count cells or evaluate quality meaningfully. Disturbing the embryo during this critical transition is generally avoided.

Some time-lapse systems can observe compaction without disturbing the embryo. Abnormal compaction patterns, such as partial compaction where only some cells participate, may predict lower blastocyst formation rates.

Day 5: blastocyst formation

By day five, the morula has developed a fluid-filled cavity called the blastocoel, and the embryo is now a blastocyst. The blastocyst has two distinct cell populations: the inner cell mass (ICM), which will become the fetus, and the trophectoderm, which will become the placenta.

Blastocyst grading typically assesses three components: expansion (how much the blastocyst has expanded and whether it has started hatching from the zona pellucida), ICM quality (graded A, B, or C based on cell number and cohesion), and trophectoderm quality (also graded A, B, or C based on cell number and organization). A common notation is something like 4AA, meaning a fully expanded blastocyst with high-quality ICM and trophectoderm.

Not all blastocysts are created equal. A 3BB blastocyst can still result in a healthy pregnancy, while a 5AA blastocyst does not guarantee one. Grading is a probability tool, not a diagnosis. The strongest predictor of whether a euploid blastocyst will implant is whether its chromosomes are normal, which is what PGT-A tests.

Day 6 and beyond: late blastocysts and freezing

Embryos that have not reached full blastocyst by day five may continue to develop on day six. Many clinics will assess day-six embryos and freeze any that have reached blastocyst stage. Some data suggests that day-six blastocysts have slightly lower implantation rates than day-five blastocysts, but the difference is modest and many day-six embryos produce healthy pregnancies.

Embryos that have not reached at least early blastocyst by day six are typically not viable and are discarded. Extending culture beyond day six is not standard practice because the culture media and conditions are optimized for five to six days. Embryos that have not developed by then are unlikely to do so.

Vitrification, the fast-freezing technique used for blastocysts, has a survival rate above 95 percent in experienced labs. Frozen-thawed blastocyst transfer outcomes are comparable to fresh transfer outcomes in most studies, and in some settings may be slightly better because the endometrium is not affected by stimulation hormones.

What the attrition funnel typically looks like

Every patient's numbers are different, but a general framework helps set expectations. For a hypothetical patient who has 12 mature eggs retrieved:

12 mature eggs retrieved. 9 fertilize normally (75 percent fertilization rate). 7 reach day three (about 80 percent of fertilized). 5 reach blastocyst (about 55 percent of fertilized). Of those 5, perhaps 2 to 3 are euploid if PGT-A is performed, depending on the patient's age.

These numbers decrease with age, particularly for the euploid proportion. A 30-year-old might see 50 to 60 percent of blastocysts test euploid. A 40-year-old might see 15 to 30 percent. The attrition funnel is the reason that egg retrieval numbers matter for cumulative probability, even though individual embryos either work or they do not.

Presenting these probabilities before the cycle begins, rather than after a disappointing result, is one sign of good clinical communication.

Evidence changes. Ask your clinician what has changed since you last reviewed your plan.

Related: Embryo Grading Explained: What the Letters and Numbers Mean