Embryo Development Day by Day: What’s Happening Behind the Scenes
Once your eggs are retrieved, the next five to six days are the most delicate parts of the IVF process: the embryo development phase. For many patients, this period can feel like a quiet stretch of waiting and wondering, but in the lab, our team is actively tracking every step of development, checking in at key milestones to assess how each embryo is progressing. Understanding what's happening in the lab during this time can help make the wait feel a little less like a mystery.
Day 0: Retrieval Day
The day of your egg retrieval is referred to as Day 0. Once the eggs are collected, they're brought into the lab and evaluated. The mature ones are then paired with sperm–whether from a partner or donor–and the fertilization process begins. Once sperm penetrates the egg, fertilization can take 12-24 hours to complete.
Day 1: Did Fertilization Occur?
On Day 1, embryologists examine each egg under a microscope to determine which has been fertilized successfully. On average, about 70–80% of mature eggs will fertilize. That said, even if an embryo appears to fertilize normally, there can still be genetic irregularities that aren't yet detectable.
Day 2-3: Early Cell Division
Over the next couple of days, fertilized eggs begin dividing. A healthy embryo will typically reach 4 cells by Day 2 and around 8 cells by Day 3. This is also the window where some embryos begin to fall behind or stop developing altogether, which is referred to as “arrest”. This happens when chromosomal abnormalities prevent development from continuing. It’s worth noting that this is a normal part of human reproduction, which can happen whether conception occurs naturally or through IVF.
Day 4: The Morula
By Day 4, a developing embryo reaches what's called the morula stage, which is a tightly compacted cluster of cells. The name comes from the Latin word for mulberry, which the embryo somewhat resembles at this point.
Day 5 (or 6): The Blastocyst
On Day 5–or sometimes Day 6 for embryos that develop a little more slowly–a successful embryo will expand into what's called a blastocyst. At this stage, two distinct cell types have formed for the first time: an inner cell mass, which has the potential to become the fetus, and an outer layer of cells, which will go on to form the placenta. Reaching the blastocyst stage is a key milestone, as this is typically when embryos are considered for transfer or freezing.
How is Embryo Quality Assessed?
Once an embryo reaches the blastocyst stage, our embryologists assign it a grade based on the timing and rate of development, as well as cell structure and organization. The grade includes a number representing the stage of development and two letters–one for the inner cell mass and one for the outer layer–on a scale from A to D, with A being the best. For example, a stage 5AB blastocyst is one that is in a later stage of development, with a high-quality inner cell mass (A) and a good-quality outer cell layer (B). These grades give us useful information about an embryo's likelihood of implanting successfully, but they are not a definitive prediction of outcome.
At Chelsea Fertility NYC, Your Embryos Are in Expert Hands
One of the hardest parts of embryo development for many patients is recognizing that this phase is largely out of their control, and that the outcome is not a reflection of anything they did or didn't do. Embryo development is a natural process that has varying rates of success both outside and inside the laboratory.
What we can do is create the best possible environment for development to unfold. That’s why at Chelsea Fertility NYC, our on-site laboratories are equipped with advanced incubation technology, supported by experienced embryologists who apply the highest standards of care and precision every day.
If you're preparing for IVF or have questions about what to expect during the embryo development phase, contact us today to learn more.
