You’ve done so much hard work. You’ve given yourself injections, gone to countless appointments, endured blood draws and ultrasounds, and made it through an egg retrieval. Now what? 

A lot has to happen for the egg and sperm to become a blastocyst, from fertilization to the various stages of embryo development. In fact, only about one in four harvested eggs may reach the blastocyst stage, and only about one in eight eggs may actually be genetically competent to become a baby (depending on your unique circumstances).

Fortunately, many experts, like Dr. Karine Chung of California Fertility Partners, empower patients to ask questions and be as informed as possible about their treatment. 

So, what does it look like for an egg to make it from fertilization to blastocyst, and how many typically get there? Let’s dive in. 

fertility doctors in a meeting

The stages of fertilization and embryo development 

During in vitro fertilization (IVF), after combining an egg and sperm, the first thing that embryologists check for is successful fertilization

After fertilization occurs, cell division begins approximately 24 hours later. During this time, the entire genome — 46 chromosomes containing more than three billion base pairs of DNA — must duplicate and divide. This is a complicated process and one reason human reproduction is not always so straightforward. 

Here’s a quick breakdown of what happens in the lab during embryo development:

chart of days 0-7 of blastocyst formation

So, how many eggs can you expect to make it to the blastocyst stage?

What about blastocysts that have a “normal” result from preimplantation genetic testing for aneuploidy (PGT-A)? The truth is, there are a lot of influencing factors to consider: 

The embryology lab

While it may seem like it’s “behind the scenes,” the embryology lab where fertilization and embryo development occur is an essential consideration for IVF success. 

Each lab has different protocols for how they culture embryos and how often they expose them to external conditions. According to Dr. Chung, the lab conditions, including embryo media and temperature control, can impact the rate at which embryos develop into blastocysts up to 10%. 

According to Dr. Chung, other questions to ask of the lab include: 

  • How are they culturing the embryos?

  • What temperature are they kept at?

  • How strictly do they adhere to the protocols?

  • How consistent are they?

While the ultimate goal is, of course, to help patients get as many blastocysts as possible, there is no set standard on how a lab grows embryos. However, experts like Dr. Chung find that embryos cultured in a controlled environment without frequent checks — mimicking the uterine environment — have higher success rates in making it to the blastocyst stage. 

In other words, not all labs have optimal blastocyst success rates, so it’s crucial to consider this when looking for the right fertility clinic for you. 

woman smiling and drinking water

Your IVF stimulation protocol

Dr. Chung reminds us that the stimulation protocol your fertility specialist recommends is also essential to IVF success, as it influences the quantity and quality of eggs retrieved. 

From medication dosages to the timing of the “trigger shot,” no protocol should be exactly the same, just as no person or pregnancy is the same. Your IVF protocol should be completely personalized to you.   

Genetics, sperm, and egg quality

One of the most common reasons an embryo fails to develop into a healthy pregnancy comes down to genetics. As you can imagine, a lot can go wrong when combining three billion DNA base pairs, and chromosomal abnormalities account for about two-thirds of embryos failing to implant or sustain life beyond the first eight weeks of pregnancy, resulting in miscarriage. 

Because female fertility declines with age, as a woman gets older we see fewer eggs retrieved during IVF, with quality also being an issue. For this reason, the chances of embryos developing into blastocysts decreases with age, especially after 40. 

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But while eggs are responsible for about 80% of IVF success, according to Dr. Chung, sperm quality plays an approximately 10% role in fertilization and the overall success of IVF as well. 

IVF is not one-size-fits-all

Now that you know more about fertilization, embryos, and blastocysts, there are a few things to think about: 

  • The age and quality of the eggs and sperm you’ll be using. 

  • Genetic testing, such as pre-implantation genetic testing for aneuploidy (PGT-A), especially if you’re over 35.

  • Finding a medical team and laboratory you trust and feel good about. 

  • Advocating for yourself! Ask questions and insist on transparency. You are the patient and deserve to be treated with patience, care, and respect. 

  • Asking questions about success rates, blastocyst development, and any other relevant information. 

  • There is no one-size-fits-all protocol in fertility treatments.

couple cuddling in front of the tv

Infertility is challenging and emotional, but there are resources, and people, in your corner. The fertilization process, embryo development, and reaching the blastocyst stage are intricate — and not always guaranteed. Only a fraction of harvested eggs reach the blastocyst stage, and even fewer possess the correct number of chromosomes to become a baby. 

However, the knowledge of contributing factors, such as the laboratory where the embryos are cultivated, the protocols followed by the clinic, the expertise of the medical professionals, egg and sperm quality, and how embryo development works, will help you navigate the process. 

The more we discuss infertility, the more questions we can answer — for others and for ourselves. The road we walk begins to have more landmarks and direction signs. We start to see friendly faces, where perhaps there were once only scary shapes and lonely vistas. 

Not every egg makes it to blastocyst. But whatever your journey through infertility, help is available to help you.

Kristin Diversi is a writer and versatile creative. She is passionate about reproductive health and justice and lives in Longmont, Colorado, with her husband and their son.