Inside the IVF Lab: Embryo Development, Genetic Testing, & What to Expect at Every Stage
About the Speakers
- Shaun Reed: Embryologist and fertility lab scientist at Utah Fertility Center who has worked in IVF since 2014. He has built a large social media following by demystifying embryology for fertility patients, giving people a rare window into what actually happens in the lab.
- Claire O’Neill: Head of Patient & Clinic Experience at Sunfish; former embryologist/andrologist with 10 years in fertility
This webinar pulls back the curtain on one of the most opaque parts of the IVF process: the embryology lab. Shaun Reed, an embryologist at Utah Fertility Center, joined Sunfish to walk through exactly what happens in the lab, from the moment eggs are retrieved to genetic testing decisions, embryo grading, and what patients should realistically expect at each stage. Click here to watch the webinar on-demand.
Key Takeaways
1. The embryology lab takes over the moment your eggs are retrieved. Embryologists don't interact much with patients directly, but their work is central to every IVF cycle. The embryology team gets notified approximately two days before a patient's egg retrieval. From the moment the physician aspirates the follicles, embryologists receive the fluid, search for eggs under a microscope, and manage the entire culture process from that point forward. Patients are often surprised to learn just how much is happening in the background.
2. IVF is a numbers game; attrition at each stage is normal and expected. Not every retrieved egg will fertilize. Not every fertilized egg will reach the blastocyst stage. Patients should expect much higher fertilization rates when using ICSI (intracytoplasmic sperm injection), and roughly 50% of fertilized eggs will develop into a usable blastocyst by day five or six based on data at UFC.
3. Embryo grading is subjective. Don't over-index on it. Embryo grades are assigned by embryologists based on visual assessment. The same embryo can receive different grades from different labs. Shaun shares that even experienced embryologists in the same room will debate the grade of the same embryo. A "BB" grade does not mean a 30% lower chance of success. If an embryologist froze a blastocyst, it met the threshold for transfer, which is the more meaningful signal.
4. PGT-A testing screens for chromosomal abnormalities, not quality. Pre-implantation genetic testing for aneuploidy (PGT-A) is a screening tool, not a guarantee. It tests whether embryos have the correct number of chromosomes. Aneuploid embryos — those with extra or missing chromosomes — are unlikely to result in a successful pregnancy and are typically set aside. PGT-A helps narrow down which embryos to transfer first, but it does not create better embryos; it only identifies which ones are chromosomally normal.
5. Mosaic embryo results are an evolving area of science — not a dead end. A mosaic PGT-A result means some cells tested as normal and some tested as abnormal. Mosaic embryos were previously considered non-transferable, but growing data shows they can result in healthy births. The success rate for mosaic embryo transfers is lower than for euploid (chromosomally normal) embryos, but it is not zero. The field is still working to understand whether mosaicism reflects the embryo itself or a limitation of current testing technology.
A Stage-by-Stage Breakdown of Embryo Development
Day 0. Egg Retrieval and Insemination: The physician performs the retrieval using a long needle and ultrasound guidance to aspirate follicles. Embryologists receive the fluid in tubes, search for eggs under a microscope, and perform insemination the same day, either through conventional IVF or ICSI.
Conventional IVF vs. ICSI:
- Conventional IVF involves adding hundreds of thousands of sperm to each drop of culture media containing eggs, allowing fertilization to occur naturally overnight.
- ICSI (intracytoplasmic sperm injection) involves manually selecting the best-looking sperm and injecting one directly into each egg. ICSI takes 30–45 minutes compared to roughly 3 minutes for conventional insemination.
Day 1: Fertilization Check: The day after egg retrieval is day one. Embryologists check each egg for fertilization by looking for two pronuclei - small circular structures inside the egg that confirm successful fertilization.
Days 2–4: Early Cell Division Most labs don't check embryos on days two or three — there's limited diagnostic value at this stage. By day three, embryos typically reach eight to ten cells. By day four, those cells compact into a structure called a morula and begin differentiating: some will become the placenta, others will become the baby.
Days 5–6: The Blastocyst Stage This is the most critical milestone. After day three, the embryo's own genome takes over development — and this is where the biggest drop-off happens. Approximately 50% of fertilized embryos are expected to reach the usable blastocyst stage by day five or six. Embryos that make it here are considered on track for transfer.
Day 7: Slow Developers Some embryos need extra time. Day seven embryos have a lower success rate than day five or six blastocysts, but they can and do result in healthy births. Most labs will culture to day seven when an embryo is still progressing and the patient has limited options.
Questions from the Audience:
Q: Why did my embryos stop developing, was it the eggs or the sperm? Unfortunately, this is rarely possible to determine with certainty using current technology. Embryologists observe development; they cannot diagnose the cause of arrest. When embryos develop well through day three and then stop, it may indicate a sperm quality contribution, since the egg typically supplies the factors needed for early division. But the honest answer, according to Shaun, is that it is likely a combination of both and IVF itself can be diagnostic in revealing that information over time.
Q: What should I know about day seven embryos? Day seven embryos are embryos that need extra time to reach the blastocyst stage beyond the standard day five or six window. They have a lower success rate than day five or day six blastocysts, but they are not without value. Many labs now routinely culture to day seven, and there is published data showing successful pregnancies from day seven embryo transfers. If a day seven embryo also comes back euploid on PGT-A testing, that is a meaningful positive signal.
Q: What Is the Success Rate for a Single Embryo Transfer? For a blastocyst-stage embryo that has been frozen and is ready for transfer, the general success rate quoted is approximately 60%, but this varies from clinic to clinic. However, that statistic is a population average — individual outcomes are influenced by the age of the egg at the time of retrieval, uterine conditions, sperm quality, and whether PGT-A was performed. A patient who transfers one embryo and gets pregnant experienced a 100% success rate on that transfer. The statistical framing can create unnecessary anxiety, particularly for patients with a limited number of embryos.
Q: Is there anything I can do to improve egg or sperm quality before an IVF cycle? According to Shaun Reed, the most honest answer is: avoid the things known to harm quality. Smoking, alcohol, and marijuana use are associated with reduced egg and sperm quality. Beyond eliminating those factors, there is limited evidence that other interventions meaningfully change biology. The embryologist's job in the lab is to maintain optimal conditions and select the best available material — not to improve on what biology has provided.
How Sunfish Supports Patients Through the IVF Process
IVF is expensive, and the lab phase — where so much of the critical work happens invisibly — can be one of the most emotionally difficult stretches of the journey. Sunfish packages IVF costs into simple monthly payment plans starting at 0% APR, eliminating the need to pay $15,000–$30,000+ upfront out of pocket. Sunfish also offers end-to-end guidance through a dedicated care team, clinic matching support, and a member perks program that gives patients access to discounted partner services throughout their cycle.
If you're navigating IVF costs and want to understand your financing options, get started at joinsunfish.com. Our care team will walk you through what a packaged IVF cycle could look like at a partner clinic near you, what monthly payment options are available, and how to build a financial plan that gets you to treatment.
This blog post is based on a Sunfish educational webinar. It is intended for informational purposes only and does not constitute medical advice. Consult your fertility specialist for guidance specific to your situation.
